Apparatus for controlling engine

ABSTRACT

An engine has variable-valve mechanisms. An engine control system has an engine control unit for executing automatic stop and start control. At an automatic-stop, the variable-valve mechanisms are controlled to obtain a valve operation characteristic suitable for a restart of the engine. When a catalyst is in an inactivated state, the variable-valve mechanisms are controlled to reduce the amount of residual gas leaking out from cylinders. At an automatic-start, the control of the variable-valve mechanism is prohibited and an intake air is adjusted by using a throttle valve. At an automatic-stop, the engine speed is abruptly reduced so that the engine speed passes through a resonant revolution speed area in a short period of time. When the voltage of a battery is low, the control of the variable-valve mechanism may be prohibited.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based on Japanese Patent Applications No.2001-372259 filed on Dec. 6, 2001, No. 2001-381015 filed on Dec. 14,2001, No. 2001-383898 filed on Dec. 18, 2001 and No. 2002-12924 filed onJan. 22, 2002 the contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an apparatus for controlling aninternal combustion engine, which is also referred to hereafter simplyas an engine. More particularly, the present invention relates to anapparatus for controlling an engine having a variable-valve mechanism.

[0004] 2. Related Art

[0005] In the conventional engine, a throttle valve is provided on anintake pipe of the engine. The throttle valve adjusts the openingthereof in order to control an intake airflow. The driver depresses anaccelerator pedal connected to the throttle valve by an electrical linkmechanism so that the valve operates in accordance with apedal-depression quantity. In addition, the throttle valve can also becontrolled by an electrical control unit and a motor. The control ofintake airflow executed by using the throttle valve is referred to asthe throttle-valve control. Since a volume exists between the throttlevalve and a cylinder, a response to a control command in the control ofthe intake airflow lags behind the command. In addition, a negativepressure is built up the downstream of the throttle valve. For thisreason, a relatively large pumping loss is incurred.

[0006] The engine also has an intake valve and an exhaust valve. Theintake valve and the exhaust valve are driven by a valve-drivingmechanism such as a cam or by an electrical actuator. Operatingcharacteristics of al least one of the intake valve and the exhaustvalve are prescribed in terms of its attributes such as an openingtiming, a closing timing, a valve opening, a valve lift quantity and alift-quantity waveform. There is known a variable-valve mechanism forvarying the operation characteristics of the valves. For example, thereis a variable-valve mechanism for adjusting the opening and the closingtimings in an advance or retard direction. Another example is avariable-valve mechanism for adjusting the opening of the valve to avalue between a zero and a maximum. Another typical variable-valvemechanism adjusts the operation characteristics with a high degree offreedom. In an engine having such a variable-valve mechanism, the intakeairflow can be adjusted by using the variable-valve mechanism. Thecontrol of intake airflow executed by the variable-valve mechanism isreferred to as variable-valve control. Typically, the operationcharacteristics of the valve are adjusted in accordance with anacceleration operation quantity and the operating state of the engine.The variable-valve control generates a small response lag in comparisonwith the throttle-valve control. In addition, in the case of thevariable-valve control, the magnitude of an incurred pumping loss can bereduced. For example, by execution of variable-valve control, thethrottle valve can be opened relatively. In a typical engine, theexecution of variable-valve control makes it unnecessary to install athrottle valve.

[0007] JP-A-8-193531 discloses an apparatus for automatically stoppingthe engine temporarily. Such an apparatus is referred to as an automaticstop and start apparatus or an idling stop control apparatus. Controlexecuted by the apparatus is known as automatic stop and start control.When the vehicle is halted, for example, the engine is automaticallystopped without the need for an operation to be carried out by thedriver. Such control is referred to as automatic stop control. When thedriver makes an attempt to drive the vehicle after the automatic stopcontrol, the engine is automatically started. In response to anoperation carried out by the driver to depress the accelerator pedal,for example, a start motor is automatically activated to start theengine automatically. The start motor can also be automaticallyactivated to start the engine when the driver carries out an operationto release the brake pedal. Such control is referred to as automaticstart control or automatic restart control. The automatic stop and startcontrol is a means capable for effectively reducing the fuelconsumption, exhaust emissions and noises.

[0008] By execution of the automatic stop and start control, on theother hand, a transient state such as the start or stop of the engineoccurs very frequently. For this reason, there is demanded propercontrol of the engine also in the transient state such as the start orstop of the engine.

[0009] Assume for example that, in automatic stop control, a valveoperation characteristic prior to the automatic stop control is saved.In this case, in the next automatic start control based on the savedvalve operation characteristic, it is feared that a smooth start of theengine is obstructed.

[0010] As another example, assume that the lift quantity of the exhaustvalve is set at a large value in automatic stop control. In this case,residual gas left in the engine flows out from the cylinder to theexhaust pipe when the engine is halted temporarily. In particular, in aninactivated state of a catalyst for cleaning exhaust gas, the state ofthe exhaust emissions is worsened.

[0011] For example, the intake change of intake airflow resulting fromthe variable-valve control can be detected by an intake-air-flow sensoror an intake-airflow meter only after a fixed delay. Right after theengine has been automatically started, on the other hand, the operatingstate of the engine changes abruptly. Thus, there is a case in which, byexecution of the variable-valve control, the intake airflow cannot beadjusted properly. As a result, a torque shock is generated. Inaddition, a change in air-fuel ratio is resulted in.

[0012] In a process wherein the engine speed becomes lower than an idlespeed due to the automatic stop control, for example, the engine speedtemporarily matches the characteristic frequency of the engine itself orthe characteristic frequency of the driving system of the vehicle. As aresult, resonance occurs, temporarily increasing the amplitude ofvibration and the magnitude of noise.

[0013] In the automatic stop control, for example, a starter is usedfrequently. As a result, there appears a tendency to reduction of thebattery voltage. In particular, the voltage of the battery decreases atan automatic-start time. When the voltage of the battery decreases, thevariable-valve mechanism does not operate in a stable manner in somecases. When the voltage of the battery decreases, for example, it isquite within the bounds of possibility that the operation characteristicof the valve cannot be controlled to follow a target operationcharacteristic. As a result, it is feared that the exhaust emissionsdeteriorate.

SUMMARY OF THE INVENTION

[0014] It is thus an object of the present invention to provide acontrol apparatus, which is capable of properly controlling an enginehaving a variable-valve mechanism when the engine is automaticallystopped.

[0015] It is another object of the present invention to provide acontrol apparatus, which is capable of properly controlling an enginehaving a variable-valve mechanism when the engine is automaticallystarted.

[0016] It is a further object of the present invention to provide acontrol apparatus, which is capable of smoothly starting an enginehaving a variable-valve mechanism when the engine is automaticallystarted.

[0017] It is a still further object of the present invention to providea control apparatus, which is capable of reducing emissions exhaustedfrom an engine having a variable-valve mechanism right after the engineis stopped.

[0018] It is a still further object of the present invention to providea control apparatus, which is capable of controlling the intake airflowof an engine having a variable-valve mechanism in a stable manner rightafter the engine is automatically stopped.

[0019] It is a still further object of the present invention to providea control apparatus, which is capable of suppressing uncomfortablevibration caused by a low speed of an engine having a variable-valvemechanism right after the engine is stopped.

[0020] It is a still further object of the present invention to providea control apparatus, which is capable of controlling the intake airflowof an engine having a variable-valve mechanism in a stable manner inautomatic start control.

[0021] In accordance with a first aspect of the present invention, rightafter an automatic stop control means automatically stops the engine, anautomatic stop valve control means computes a target valve operationcharacteristic on the basis of the present state of the engine and/orthe present state of the vehicle, and controls a valve operationcharacteristic to the target valve operation characteristic for anautomatic-stop time.

[0022] An automatic-stop time is defined as a time between an automaticstop of the engine and an automatic start of the engine. In general, theautomatic-stop time such as a time of waiting for a traffic light toturn to a green color is relatively short in many cases. Thus, while theengine is in an automatically stopped state, the state of the engineand/or the state of the vehicle such as the temperature of the coolingwater do not change much in many cases. Accordingly, it is possible tofind a valve operation characteristic in which the state of the engineand/or the state of the vehicle at an automatic start of the engineafter an automatic stop of the engine are the same as the state of theengine and/or the state of the vehicle right after the automatic stop sothat the state of the engine and/or the state of the vehicle right afterthe automatic stop can be applied to the automatic start after theautomatic stop.

[0023] Thus, right after the engine is automatically stopped, it ispossible to find a valve operation characteristic, which is presumed tobe proper for an automatic start after the automatic stop from thepresent state of the engine and/or the present state of the vehicleright after the automatic stop, as a target valve operationcharacteristic, and control the valve operation characteristic to thetarget valve operation characteristic while the engine is in anautomatically stopped state. At the next automatic-start time, theengine can be automatically started under a valve operationcharacteristic approximately proper for an automatic start so that anautomatic-start characteristic of the engine can be improved and exhaustemissions at the automatic-start time can be reduced.

[0024] By the way, if an exhaust valve of any cylinder is largely openedin an automatically stopped state of the engine, resulting in a state inwhich residual gas remaining in the cylinder leaks out to an exhaustpipe with ease, it is quite within the bounds of possibility that theresidual gas leaking out from the cylinder is discharged to theatmosphere without being cleaned by a catalyst provided on the exhaustpipe as a means for cleaning exhaust gas provided that the catalyst isin a pre-warmed state or an inactivated state.

[0025] In order to solve the above problem, if the catalyst is presumedto be in a state of being warmed or activated insufficiently on thebasis of the present state of the engine and/or the present state of thevehicle, which is detected right after the engine is stoppedautomatically, a valve operation characteristic making residual gas leftin a cylinder difficult to leak out is found and set as a target valveoperation characteristic for an automatic-stop time. An example of sucha condition is a condition in which the lift quantity of the valve is 0or a minimum. If the catalyst is presumed to be in a state of beingwarmed or activated insufficiently, the engine can be stopped into anautomatic stopped state by using a valve operation characteristic makingresidual gas left in a cylinder difficult to leak out as a target valveoperation characteristic for the automatic stop. Thus, exhaust emissionscan be reduced during an automatic stop.

[0026] When an automatic start control means automatically starts theengine, an automatic start valve control means computes a target valveoperation characteristic on the basis of the present state of the engineand/or the present state of the vehicle, and controls a valve operationcharacteristic to the target valve operation characteristic for anautomatic start time. When the engine is automatically started, a targetvalve operation characteristic optimum for an automatic start is foundon the basis of the present state of the engine and/or the present stateof the vehicle, and used as a target valve operation characteristic whenthe engine is automatically started. Thus, at an automatic-start time ofthe engine, the engine can be automatically started under a target valveoperation characteristic optimum for an automatic start. As a result, anautomatic-start characteristic of the engine can be improved and exhaustemissions at the automatic-start time can be reduced.

[0027] Right after an automatic stop of the engine, a target valveoperation characteristic for an automatic-stop time is found on thebasis of the present state of the engine and/or the present state of thevehicle, and the valve operation characteristic is controlled to thetarget valve operation characteristic for the automatic-stop time. Inaddition, a target valve operation characteristic for an automatic-starttime is found on the basis of the present state of the engine and/or thepresent state of the vehicle, and the valve operation characteristic iscontrolled to the target valve operation characteristic for theautomatic-start time.

[0028] In this configuration, right after an automatic stop of theengine, a target valve operation characteristic for an automatic-stoptime is found on the basis of the present state of the engine and/or thepresent state of the vehicle, and the valve operation characteristic iscontrolled in advance for the time being to the target valve operationcharacteristic for the automatic-stop time. In addition, a target valveoperation characteristic for an automatic-start time is found on thebasis of the present state of the engine and/or the present state of thevehicle, and the valve operation characteristic is controlled to thetarget valve operation characteristic for the automatic-start time. Whenthe engine is automatically started, the magnitude of correction of thevalve operation characteristic can be reduced so that the valveoperation characteristic can be corrected to a valve operationcharacteristic optimum for an automatic start in a short period of time.In addition, even if the valve operation characteristic set during theautomatic stop is inevitably shifted from the valve operationcharacteristic optimum for the current automatic start due to a largechange in engine state and/or a change in vehicle state during theautomatic stop, the valve operation characteristic can be corrected to avalve operation characteristic optimum for an automatic start at anautomatic-start time.

[0029] It is to be noted that, if the catalyst is presumed to be in astate of being warmed or activated insufficiently on the basis of thestate of the engine and/or the state of the vehicle right after anautomatic stop of the engine, right after the automatic stop of theengine, first of all, the valve operation characteristic is controlledin advance to a valve operation characteristic making residual gas leftin a cylinder difficult to leak out and, then, when the engine isautomatically started, the valve operation characteristic can becorrected to a valve operation characteristic optimum for an automaticstart. An automatic-start characteristic of the engine can be improvedand, at the same time, exhaust emissions at the automatic stop of theengine can be reduced.

[0030] By the way, in general, the lower the temperature of the engineand/or the lower the temperature of the battery mounted on the vehicle,the poorer the performance of the battery. The poorer the performance ofthe battery, the smaller the driving power of a starter. The smaller thedriving power of a starter, the lower the flowability of the engine oil.The lower the flowability of the engine oil, the greater the frictionsamong movable parts. Thus, the cranking of the automatic-start time isprone to variations and the automatic-start characteristic of the enginetends to deteriorate. In addition, the number of automatic stops or thenumber of automatic starts increases and the automatic-stop time islengthened so that the consumption of the battery power during anautomatic stop rises. With the increased consumption of the batterypower during an automatic stop, the start power decreases due to theconsumption of power from the battery, and the automatic-startcharacteristic of the engine tends to deteriorate.

[0031] A target valve operation characteristic for an automatic-stoptime can be found on the basis of at least one of an automatic-stopcount (or the number of previous automatic stops or the number ofautomatic stops carried out so far, a cooling-water temperature, anintake-air temperature, an oil temperature and pieces of informationhaving correlations with the automatic-stop count, the cooling-watertemperature, the intake-air temperature and the oil temperature. By theautomatic-stop count, the number of previous automatic stops or thenumber of automatic stops carried out so far is meant. As analternative, a target valve operation characteristic for anautomatic-start time is found on the basis of at least one of anautomatic-stop count, an automatic-stop time, a cooling-watertemperature, an intake-air temperature, an oil temperature and pieces ofinformation having correlations with the automatic-stop count, theautomatic-stop time, the cooling-water temperature, the intake-airtemperature and the oil temperature. If a target valve operationcharacteristic for an automatic-stop time and/or a target valveoperation characteristic for an automatic-start time are found using atleast one of information for determining a warming state of the engine(that is, temperatures of the engine such as a cooling-watertemperature, an intake-air temperature and an oil temperature) andinformation for determining the performance of the battery such as theautomatic-stop count and the automatic-stop time, the valve operationcharacteristic can be controlled in a direction of stabilizing thecranking of the engine occurring at an automatic-start time in order tocope with a state of easy-to-occur cranking variations caused by areduced driving power of the starter and increased frictions amongmovable components. As a result, the automatic-start characteristic ofthe engine can be further improved. The reduced driving power of thestarter is attributed to the deterioration of performance of batteryoccurring at a low temperature of the engine and/or a low temperature ofthe battery. An example of the direction of stabilizing the cranking ofthe engine is a direction of increasing the intake airflow.

[0032] When an engine stall occurs due to a failure of an automaticstart of the engine, the valve operation characteristic can becontrolled in a direction of increasing the intake airflow prior to arestart of the engine. In an operation to start the engine, the enginecan be restarted with an intake airflow greater than the valve operationcharacteristic for an engine-stall state after the engine stall due to afailure of an automatic start of the engine. Thus, the engine stall isprevented from being generated several times consecutively. As a result,the engine can be restarted successfully at an early time.

[0033] In accordance with a second aspect of the present invention, avariable-valve control prohibition means fixes the valve operationcharacteristic at a predetermined valve operation characteristic duringa predetermined period after an automatic start of the engine, and athrottle-valve control means controls the opening of a throttle valveprovided on the intake pipe of the engine in order to adjust the intakeairflow. The predetermined period is referred to hereafter as avariable-valve control prohibit period.

[0034] In this configuration, during the variable-valve control prohibitperiod, that is, during a period including complicated and much variabletransient times following an automatic start of the engine, the valveoperation characteristic is fixed and the variable-valve control foradjusting the intake airflow is prohibited. Instead, throttle-valvecontrol is executed to adjust the intake airflow. In comparison with thevariable-valve control, the throttle-valve control exhibits a smalldelay of detection of an intake airflow at a transient time. Thus,during a period of an unstable operating state following an automaticstart of the engine, the throttle-valve control is executed to stabilizethe intake airflow so that it is possible to prevent the drivabilityfollowing an automatic start of the engine and exhaust emissionsfollowing the automatic start of the engine from deteriorating.

[0035] In this case, if a difference between a target valve operationcharacteristic set initially at an automatic start of the engine and avalve operation characteristic fixed during the variable-valve controlprohibit period following the completion of the automatic start of theengine is large, the valve operation characteristic prior to thecompletion of the automatic start of the engine greatly changes in anabrupt manner to a valve operation characteristic after the completionof the automatic start of the engine so that it is quite within thebounds of possibility that the abrupt change in valve operationcharacteristic appears as a torque shock and/or a deterioration ofexhaust emissions.

[0036] In order to solve the above problem, during the variable-valvecontrol prohibit period following the completion of the automatic startof the engine, the valve operation characteristic is fixed at a targetvalve operation characteristic for an automatic-start time of theengine. Since the valve operation characteristic is sustained and fixedprior to and after the completion of the automatic start of the engine,variations in valve operation characteristic can be eliminated. Thus, atorque shock and/or deterioration of exhaust emissions can be preventedfrom occurring due to an abrupt change in valve operationcharacteristic.

[0037] In addition, while the variable-valve control prohibit periodfollowing the automatic start of the engine can be set at a fixed valuedetermined in advance, the variable-valve control prohibit periodfollowing the automatic start of the engine can be set at a valuedependent on the number of previous engine automatic stops or the numberof previous engine automatic starts. If the number of previous engineautomatic stops after a start of a vehicle run or the number of previousengine automatic starts after the start of the vehicle run is small, thenumber of times an adverse effect is experienced can be determined to besmall as well. Examples of the adverse effect are deteriorations causedby the variable-valve control such as a deterioration of the drivabilityand a deterioration of exhaust emissions. Since the number of times anadverse effect is experienced is small, the variable-valve controlprohibit period can be shortened and the variable-valve control can thusbe started at an early time after the automatic start of the engine.Thus, control to let the improvement of the performance take precedenceof others can be executed. Typically, the performance such as fueleconomy can be improved by execution of the variable-valve control. Ifthe number of previous engine automatic stops after a start of a vehiclerun or the number of previous engine automatic starts after the start ofthe vehicle run is large, on the other hand, the number of times anadverse effect is experienced can be determined to be large as well. Inthis case, the variable-valve control prohibit period is lengthened.Thus, control can be executed to let avoidance of the adverse effectcaused by the variable-valve control take precedence of others ratherthan letting the improvement of the performance take precedence ofothers.

[0038] In accordance with a third aspect of the present invention, thereis provided a variable-valve mechanism capable of controlling the intakeairflow by varying valve operation characteristics of the intake valveor exhaust valve or both the valves of the engine. An intake airflow iscontrolled by adjusting the variable-valve mechanism and/or a throttlevalve so as to gradually reduce a torque output by the engine and stopthe engine when a predetermined condition for an automatic stop of theengine is satisfied during an operation of the engine. In addition,during the process to reduce the torque output by the engine, torqueabrupt reduction control is executed to abruptly decrease the intakeairflow by controlling the variable-valve mechanism so as to abruptlyreduce the torque output by the engine with a timing with which theengine speed is about to pass through a predetermined speed zone. Inthis case, it is preferable to set the predetermined speed zone forexecution of the torque abrupt reduction control to include a resonancespeed area in which vibration of the engine is resonant with vibrationof a vehicle-driving system.

[0039] Thus, when the engine is automatically stopped, if thevariable-valve mechanism is controlled to abruptly decrease the intakeairflow with a timing with which the engine speed is about to passthrough the predetermined speed zone including the resonance speed area,the intake airflow into a cylinder abruptly decreases, exhibiting goodresponsiveness also with the timing with which the engine speed is aboutto pass through the predetermined speed zone. Thus, the engine speed canbe reduced abruptly, passing through the predetermined speed zoneincluding the resonance speed zone. As a result, at the time of theautomatic stop control, the engine speed can be changed through theresonance speed zone in a short period of time so that noises andvibration, which are caused by the resonance phenomenon, can be reducedwith a high degree of reliability without making the driver feel a senseof incompatibility.

[0040] In the case of a system having a variable-valve mechanism capableof controlling an intake valve to a completely closed state or a statewith a valve lift quantity of 0, it is preferable to control thevariable-valve mechanism to put the intake valve in the completelyclosed state at the time of the torque abrupt reduction control. At thetime of the torque abrupt reduction control, it is possible to reducethe intake airflow into a cylinder to 0 instantaneously and, hence, toabruptly decrease the engine speed. Thus, the engine speed can bechanged to pass through the resonance speed zone in a short period oftime. As a result, noises and vibration, which are caused by theresonance phenomenon, can be reduced substantially.

[0041] In the case of a system having a variable-valve mechanism notcapable of controlling an intake valve to a completely closed state, onthe other hand, it is preferable to control the variable-valve mechanismto minimize the intake valve at the time of the torque abrupt reductioncontrol and to control a throttle valve to a completely closed state.Even in the case of a system having a variable-valve mechanism notcapable of controlling an intake valve to a completely closed state,both the variable-valve control and the throttle-valve control areeffectively executed at the time of the torque abrupt reduction controlto set the intake airflow at 0 quickly in order to reduce the enginespeed abruptly. Thus, in the case of a variable-valve mechanism notcapable of controlling an intake valve to a completely closed state, theengine speed can be changed to pass through the resonance speed zone inan extremely short period of time. As a result, noises and vibration,which are caused by the resonance phenomenon, can be reducedeffectively.

[0042] In addition, injection of fuel can also be stopped at the time ofthe torque abrupt reduction control. Thus, the engine speed can beabruptly reduced with a high degree of effectiveness by reducing theintake airflow as well as stopping the injection of fuel at the time ofthe torque abrupt reduction control.

[0043] In addition, in a process to gradually reduce a torque output bythe engine and stop the engine at the time of the automatic-stopcontrol, the fuel injection volume can be controlled so as to maintainan air-fuel ratio at a target air-fuel ratio till the engine speed isreduced to a predetermined speed zone. The air-fuel ratio can bemaintained at the target air-fuel ratio in a process to gradually reducethe torque output by the engine at the time of the automatic-stopcontrol. Thus, it is possible to gradually reduce the engine speedwithout deteriorating exhaust emissions.

[0044] In accordance with a fourth aspect of the present invention,there is provided a variable-valve control prohibition means forprohibiting control executed by a variable-valve control means to openand close an intake valve and/or an exhaust valve on the basis of abattery voltage detected by a battery-voltage-driving means after theengine is automatically started by an automatic-start control means.

[0045] Thus, if the voltage of a battery decreases after the engine isautomatically started so that the control response characteristics ofthe intake valve and/or the exhaust valve deteriorate, making itimpossible to follow target valve lift quantities and follow valveopening/closing timings with a high degree of precision, the control ofthe intake valve and/or the exhaust valve is prohibited and, instead,the valve positions are fixed so as to stabilize a target intake airflowand, hence, suppress deteriorations of exhaust emissions.

[0046] In addition, it is preferable to have the variable-valve controlprohibition means prohibit intake-air-flow control executed by using theintake valve and/or the exhaust valve till the voltage of the batteryreaches a predetermined level.

[0047] In control of the intake airflow by varying a lift quantityvariable through the use of electric power, in particular, the intakeairflow at a location in close proximity to a combustion chamber of theengine can be adjusted. It is thus unnecessary to take a delay of an airsystem into consideration in comparison with the intake-airflow controlby using a throttle valve. As a result, the control of the intakeairflow can be executed with a high degree of precision. When thevoltage of the battery decreases, however, a response delay is incurredin the control of the intake valve and/or the exhaust valve so that theprecision of the control of the intake airflow and the exhaust emissionsinevitably deteriorate. Thus, when the voltage of the battery decreases,the valve lift quantity is held at a fixed value and the control of theintake airflow is prohibited in order to suppress the deteriorations ofthe exhaust emissions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Features and advantages of embodiments will be appreciated, aswell as methods of operation and the function of the related parts, froma study of the following detailed description, the appended claims, andthe drawings, all of which form a part of this application. In thedrawings:

[0049]FIG. 1 is a block diagram of the configuration of an enginecontrol system according to a first embodiment of the present invention;

[0050]FIG. 2 is a diagram of the configuration of a variable valveaccording to the first embodiment of the present invention;

[0051]FIG. 3 is a graph representing a state of a large lift quantity ofa variable-valve mechanism according to the first embodiment of thepresent invention;

[0052]FIG. 4 is a graph representing a state of a small lift quantity ofthe variable-valve mechanism according to the first embodiment of thepresent invention;

[0053]FIG. 5 is a graph representing operation characteristics of thevariable-valve mechanism according to the first embodiment of thepresent invention;

[0054]FIG. 6 is a flowchart representing engine control according to thefirst embodiment of the present invention;

[0055]FIG. 7A is a graph representing relations between an enginecooling water temperature Tw and a basic valve lift quantity Bstop inthe first embodiment of the present invention;

[0056]FIG. 7B is a graph representing a relation between anengine-automatic-stop count NS or an engine-automatic-start count NR anda valve-lift-quantity correction coefficient Cstop in the firstembodiment of the present invention;

[0057]FIG. 8 is a flowchart representing other engine control accordingto the first embodiment of the present invention;

[0058]FIG. 9A is a graph representing other relations between the enginecooling water temperature Tw and the basic valve lift quantity Bstart inthe first embodiment of the present invention;

[0059]FIG. 9B is a graph representing another relation between theengine-automatic-stop count NS or the engine-automatic-start count NRand a first valve-lift-quantity correction coefficient C1start in thefirst embodiment of the present invention;

[0060]FIG. 9C is a graph representing a relation between anengine-automatic-stop time Ts and a second valve-lift-quantitycorrection coefficient C2start in the first embodiment of the presentinvention;

[0061]FIG. 10 is a flowchart representing further engine controlaccording to the first embodiment of the present invention;

[0062]FIG. 11 is a graph representing a relation between an engine-stallcount Nes and a valve lift quantity increase ΔVL in the first embodimentof the present invention;

[0063]FIG. 12 is a time chart representing engine control according tothe first embodiment of the present invention;

[0064]FIG. 13 is a time chart representing other engine controlaccording to the first embodiment of the present invention;

[0065]FIG. 14 is a time chart representing further engine controlaccording to the first embodiment of the present invention;

[0066]FIG. 15 is a flowchart representing engine control according to asecond embodiment of the present invention;

[0067]FIG. 16 is a graph representing a relation between theengine-automatic-stop count NS or the engine-automatic-start count NRand a prohibition time KCAST of variable-valve control in the secondembodiment of the present invention;

[0068]FIG. 17 is a time chart representing the engine control accordingto the second embodiment of the present invention;

[0069]FIG. 18 is a flowchart representing engine control according to athird embodiment of the present invention;

[0070]FIG. 19 is a time chart representing the engine control accordingto the third embodiment of the present invention;

[0071]FIG. 20 is a flowchart representing engine control according to afourth embodiment of the present invention;

[0072]FIG. 21 is a flowchart representing other engine control accordingto the fourth embodiment of the present invention;

[0073]FIG. 22 is a map for setting a target intake airflow excessreduction quantity FQA in the fourth embodiment of the presentinvention;

[0074]FIG. 23 is a map for setting a target lift quantity VL in thefourth embodiment of the present invention;

[0075]FIG. 24 is a flowchart representing further engine controlaccording to the fourth embodiment of the present invention;

[0076]FIG. 25 is a flowchart representing still further engine controlaccording to the fourth embodiment of the present invention;

[0077]FIG. 26 is a time chart representing engine control according tothe fourth embodiment of the present invention; and

[0078]FIG. 27 is a graph representing a relation between an engine speedNE and the magnitude of a noise in the fourth embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0079] First Embodiment

[0080] Some preferred embodiments of the present invention are explainedby referring to diagrams as follows. First of all, a rough configurationof an entire engine control system is explained by referring to FIG. 1.An air cleaner 13 is provided at the upper end of the upstream side ofan intake pipe 12 employed in an internal combustion engine 11. Anairflow meter 14 for detecting an intake airflow is provided thedownstream of the air cleaner 13. Downstream of the airflow meter 14,there are provided a throttle valve 15, the opening of which can beadjusted by typically a DC motor, and a throttle-opening sensor 16 fordetecting an opening of the throttle valve 15.

[0081] A surge tank 17 is further provided downstream of the throttlevalve 15. On the surge tank 17, there is provided anintake-pipe-pressure sensor 18 for detecting a pressure of air in theintake pipe 12. In addition, on the surge tank 17, there is provided anintake manifold 19 for introducing air into cylinders employed in theengine 11. At locations in close proximity to an intake port of theintake manifold 19, there are provide fuel injection valves 20 forinjecting fuel into their respective cylinders. Ignition plugs 21 eachprovided for one of the cylinders are installed on cylinder heads of theengine 11. Mixed gas in a cylinder is ignited by a spark electricdischarge of the ignition plug 21 provided for the cylinder.

[0082] On an intake valve 28 employed in the engine 11, there isprovided a variable-valve lift mechanism 30 for changing the liftquantity of the intake valve 28. By the same token, on an exhaust valve29 employed in the engine 11, there is provided a variable-valve liftmechanism 31 for changing the lift quantity of the exhaust valve 29. Inaddition, on the intake valve 28, it is possible to provide avariable-valve lift mechanism for changing the valve timing of theintake valve 28. In the same way, on the exhaust valve 29, it ispossible to provide a variable-valve lift mechanism for changing thevalve timing of the exhaust valve 29.

[0083] On the other hand, on an exhaust pipe 22 employed in the engine11, there is provided a catalyst 23 such as a 3-way catalyst forreducing the amounts of emissions such as CO, HC and NOx contained inexhaust gas. Upstream of the catalyst 23, there is provided anair-fuel-ratio sensor 24 such as a linear air-fuel-ratio sensor or anoxygen sensor for detecting an air fuel ratio of exhaust gas ordetermining whether the air fuel ratio is on the rich or lean side. Inaddition, on a cylinder block of the engine 11, there are provided acooling-water-temperature sensor 25 for detecting a temperature ofcooling water and a crank-angle sensor 26 for detecting an engine speed.

[0084] Signals generated by these sensors are supplied to an enginecontrol circuit 27 referred to hereafter as an ECU. The ECU 27 has aconfiguration including a microcomputer as a core component. Themicrocomputer executes a variety of control programs stored in anembedded ROM, which serves as a storage medium, in order to control fuelinjection volumes of the fuel injection valves 20 and ignition timingsof the ignition plugs 21 in accordance with an operating state of theengine 11.

[0085] Next, the configuration of the variable-valve mechanism 30 of theintake valve 28 is explained by referring to FIGS. 2 to 5. It is to benoted that, since the configuration of the variable-valve mechanism 31of the exhaust valve 29 is the same as the configuration of thevariable-valve mechanism 30 of the intake valve 28, the configuration ofthe variable-valve mechanism 31 is not explained specially.

[0086] As shown in FIG. 2, a link arm 34 is provided between a rockerarm 33 and a cam shaft 32 for driving the intake valve 28. Above thelink arm 34, there is provided a control shaft 35 rotated by a steppingmotor not shown in the figure. On the control shaft 35, there isprovided an eccentric cam 36 rotatable with the control shaft 35 as asingle body. The link arm 34 is supported at an eccentric positionrelative to the axis of the eccentric cam 36 by a support shaft notshown in the figure in such a manner that the link arm 34 can bereciprocated. A reciprocating cam 38 is provided at the center of thelink arm 34. A side surface of the reciprocating cam 38 is in contactwith an outer circumferential surface of a cam 37 provided on the camshaft 32. A pressure cam 39 is provided on the lower end of the link arm34. The lower-end surface of the pressure cam 39 is in contact with theupper-end surface of a roller 40 provided at the center of the rockerarm 33.

[0087] With the above configuration, when the cam 37 is rotated by therotation of the cam shaft 32, the reciprocating cam 38 of the link arm34 reciprocates horizontally in accordance with the outercircumferential shape of the cam 37, causing the link arm 34 to alsoreciprocate horizontally as well. When the link arm 34 reciprocateshorizontally, the pressure cam 39 also reciprocates horizontally so thatthe roller 40 of the rocker arm 33 moves up and down in accordance withthe lower-end surface shape of the pressure cam 39, causing the rockerarm 33 also to move up and down as well. When the rocker arm 33 moves upand down, the intake valve 28 also moves up and down as well.

[0088] When the eccentric cam 36 is rotated by the rotation of thecontrol shaft 35, on the other hand, the position of the support shaftof the link arm 34 moves, changing an initial contact point positionbetween the pressure cam 39 of the link arm 34 and the roller 40 of therocker arm 33. For the initial contact point position, refer to FIGS. 3and 4. In addition, as shown in FIG. 2, the lower-end surface of thepressure cam 39 of the link arm 34 comprises a base surface 39 a formedat such a curvature that the magnitude of a pressure of the rocker arm33 at a left-side portion is 0, that is, the valve lift quantity of theintake valve 28 is 0, and a base surface 39 b formed at such a curvaturethat the magnitude of a pressure of the rocker arm 33 increases whenmoving in the right direction starting from the base surface 39 a, thatis, the valve lift quantity of the intake valve 28 increases when movingin the direction.

[0089] In a large lift mode in which the valve lift quantity of theintake valve 28 is increased, the rotation of the control shaft 35 movesthe initial contact point position between the pressure cam 39 of thelink arm 34 and the roller 40 of the rocker arm 33 in the rightdirection as shown in FIG. 3. Thus, when the pressure cam 39 isreciprocated horizontally due to the rotation of the cam 37, aparticular portion of the lower-end surface of the pressure cam 39 ismoved to the right. Accordingly, the largest magnitude of a pressure ofthe rocker arm 33 increases, raising the largest valve lift quantity ofthe intake valve 28 and lengthening a period in which the rocker arm 33is pressed. As a result, an opened-valve period of the intake valve 28is also lengthened as well. By the particular portion, the portion oflower-end surface in contact with the roller 40 is meant.

[0090] In a small lift mode in which the valve lift quantity of theintake valve 28 is decreased, on the other hand, the rotation of thecontrol shaft 35 moves the initial contact point position between thepressure cam 39 of the link arm 34 and the roller 40 of the rocker arm33 in the left direction as shown in FIG. 4. Thus, when the pressure cam39 is reciprocated horizontally due to the rotation of the cam 37, aparticular portion of the lower-end surface of the pressure cam 39 ismoved to the left. Accordingly, the largest magnitude of a pressure ofthe rocker arm 33 decreases, reducing the largest valve lift quantity ofthe intake valve 28 and shortening a period in which the rocker arm 33is pressed. As a result, an opened-valve period of the intake valve 28is also shortened as well. By the particular portion, the portion oflower-end surface in contact with the roller 40 is meant as describedabove.

[0091] In the variable-valve lift mechanism 30 described above, if theinitial contact point position between the pressure cam 39 of the linkarm 34 and the roller 40 of the rocker arm 33 is moved continuously byrotating the control shaft 35 by using the stepping motor, it ispossible to continuously change the largest valve lift quantity of theintake valve 28 and the opened-valve period of the intake valve 28 asshown in FIG. 5.

[0092] Driven by power generated by a battery 41 mounted on the vehicle,the ECU 27 executes a variable-valve lift control program stored in theROM, controlling the variable-valve lift mechanism 30 of the intakevalve 28 and the variable-valve lift mechanism 31 of the exhaust valve29 on the basis of an accelerator position, an operating state of theengine 11 and other information in order to continuously change thevalve lift quantities of the intake valve 28 and the exhaust valve 29.In this case, the ECU 27 functions as a variable-valve control means forcontrolling the intake airflow. It is to be noted that, in a systememploying a variable valve timing mechanism in conjunction with thevariable-valve lift mechanisms 30 and 31, both the valve lift quantitiesand the valve timings may be continuously changed in order to controlthe intake airflow.

[0093] In addition, the ECU 27 executes the automatic-stop controlprogram of ROM shown in FIG. 6 to automatically stop the engine 11 if apredetermined automatic-stop condition is satisfied during an operationof the engine 11. Right after the automatic stop, the ECU 27 finds atarget valve lift quantity VLstop of an engine automatic-stop time forthe intake valve 28 and a target valve lift quantity VLstop of an engineautomatic-stop time for the exhaust valve 29 on the basis of a state ofthe engine 11 and a state of the vehicle, controlling the valve liftquantities of the intake valve 28 and the exhaust valve 29 to theirrespective target valve lift quantities VLstop. The target valve liftquantity VLstop is a valve lift quantity presumed to be suitable for thenext automatic start of the engine 11 or a valve lift quantity making itdifficult for residual gas left in the cylinders to leak out. At a pointof time the valve lift quantities of the intake valve 28 and the exhaustvalve 29 become equal to their respective target valve lift quantitiesVLstop, the control of the variable-valve lift mechanisms 30 and 31 isdiscontinued.

[0094] Furthermore, the ECU 27 executes the automatic-start controlprogram of ROM shown in FIG. 8 to first find a target valve liftquantity VLstart of an engine automatic-start time for the intake valve28 and a target valve lift quantity VLstart of an engine automatic-starttime for the exhaust valve 29 on the basis of a state of the engine 11and a state of the vehicle, controlling the valve lift quantities of theintake valve 28 and the exhaust valve 29 to their respective targetvalve lift quantities Vlstart if a predetermined automatic-startcondition is satisfied in an automatic-stop state of the engine 11. Thetarget valve lift quantity VLstart is a valve lift quantity optimum foran automatic start of the engine 11. Then, at a point of time the valvelift quantities of the intake valve 28 and the exhaust valve 29 becomeequal to their respective target valve lift quantities VLstart, the ECU27 automatically starts the engine 11.

[0095] Moreover, the ECU 27 executes the engine-stall-generation-timecontrol program of ROM shown in FIG. 10 to correct a target valve liftquantity VLstart of an engine automatic-start time for the intake valve28 in a direction of increasing an intake airflow in the event of theso-called engine stall caused by a failure of an automatic start of theengine 11, and control the valve lift quantity of the intake valve 28 tothe corrected target valve lift quantity VLstart of an engineautomatic-start time for the intake valve 28. Then, at a point of timethe valve lift quantity of the intake valve 28 becomes equal to thecorrected target valve lift quantity VLstart of an engineautomatic-start time for the intake valve 28, the ECU 27 automaticallyrestarts the engine 11.

[0096] The following description explains the processing of the controlprograms executed by the ECU 27 by referring to flowcharts shown inFIGS. 6, 8 and 10.

[0097] Automatic-Stop Control

[0098] The automatic-stop control program represented by the flowchartshown in FIG. 6 is executed repeatedly at predetermined time intervalsduring the operation of the engine 11. When this program is invoked, theflowchart begins with a step 101 to determine whether or notautomatic-stop conditions are satisfied. Typically, the automatic-stopconditions include conditions (1) to (3) described as follows.

[0099] (1): The speed of the vehicle shall be 0 km/h. That is, thevehicle shall be in a stopped state.

[0100] (2): The accelerator pedal shall not be depressed.

[0101] (3): The brake pedal shall be in a state of being depressed.

[0102] If conditions (1) to (3) are all satisfied, the automatic-stopconditions are considered to hold true. If even only one of conditions(1) to (3) is not satisfied, on the other hand, the automatic-stopconditions are considered not to hold true. It is to be noted that theautomatic-stop conditions can be modified if necessary.

[0103] If the automatic-stop conditions are satisfied during theoperation of the engine 11, a request for an engine stop is determinedto exist. In this case, the flow of the program goes on to a step 102 atwhich automatic stop control or idling stop control is executed. In thisautomatic stop control, a fuel cut operation and an ignition cutoperation are carried out to automatically stop the engine 11. Theprocessing of the step 102 is carried out to play the role of anautomatic stop control means.

[0104] Then, the flow of the program goes on to a step 103 to determinewhether or not the automatic stop of the engine 11 has been completed byfor example determining whether or not the engine speed NE has decreasedto 0. At a point of time the automatic stop of the engine 11 iscompleted, the flow of the program goes on to a step 104 at which targetvalve lift quantities VLstop of the intake valve 28 and the exhaustvalve 29 for the engine automatic-stop time are each computed inaccordance with an equation given below. As described above, the targetvalve lift quantity VLstop is a valve lift quantity presumed to besuitable for the next automatic start of the engine 11 or a valve liftquantity making it difficult for residual gas left in the cylinders toleak out.

VLstop=Bstop×Cstop

[0105] where reference notation Bstop is a basic valve lift quantity forthe engine automatic-stop time and reference notation Cstop is avalve-lift-quantity correction coefficient for correcting the basicvalve lift quantity Bstop.

[0106] A basic valve lift quantity Bstop is set in dependence on anengine-cooling-water temperature detected right after the automatic stopof the engine 11 by using a formula or a map prepared for the basicvalve lift quantity Bstop for the engine automatic-stop time like oneshown in FIG. 7A.

[0107] In accordance with the map of basic valve lift quantity Bstopshown in FIG. 7A, in a zone where the engine-cooling-water temperaturedetected right after the automatic stop of the engine 11 is lower than apredetermined value and the catalyst 23 can be assumed to be in aninactivated state, the basic valve lift quantity Bstop used as a basevalue for the target valve lift quantity VLstop for the engineautomatic-stop time is set at 0 or a minimum in order to attachimportance to reduction of exhaust emissions during the automatic stopof the engine 11 and to set the target valve lift quantity VLstop at avalve lift quantity making it difficult for residual gas left in thecylinders to leak out during the automatic stop of the engine 11.

[0108] In accordance with the map of basic valve lift quantity Bstopshown in FIG. 7A, in a zone where the engine-cooling-water temperaturedetected right after the automatic stop of the engine 11 is at leastequal to the predetermined value and the catalyst 23 can be assumed tobe in an activated state, on the other hand, the basic valve liftquantity Bstop used as a base value for the target valve lift quantityVLstop for the engine automatic-stop time is set in accordance with anengine-cooling-water temperature Tw detected right after the automaticstop in order to attach importance to the next automatic startcharacteristic or the restart characteristic of the engine 11 and to setthe target valve lift quantity VLstop at a valve lift quantity presumedto be proper for the next automatic start of the engine 11 from astandpoint of the engine-cooling-water temperature Tw detected rightafter the automatic stop.

[0109] In general, the lower the temperature of the engine and/or thelower the temperature of the battery mounted on the vehicle, the poorerthe performance of the battery. The poorer the performance of thebattery, the smaller the driving power of a starter not shown in thefigure. The smaller the driving power of the starter, the lower theflowability of the engine oil. The lower the flowability of the engineoil, the greater the frictions among movable parts. Thus, the crankingof the automatic-start time is prone to variations and theautomatic-start characteristic of the engine tends to deteriorate. Sincethe battery is mounted in the same room as the engine 11, thetemperature of the battery changes due to heat dissipated by the engine11 in accordance with the temperature of the engine 11. From thisrelation, when the engine cooling-water temperature representing thetemperature of the engine 11 is low, the temperature of the engine 11can be presumed to be also low as well.

[0110] In accordance with the map of basic valve lift quantity Bstopshown in FIG. 7A, in the zone where the engine-cooling-water temperaturedetected right after the automatic stop of the engine 11 is at leastequal to the predetermined value and the catalyst 23 can be assumed tobe in an activated state, the lower the engine cooling-water temperaturerepresenting the temperature of the engine 11, the larger the value atwhich the basic valve lift quantity Bstop is set. Thus, in order to copewith the fact that the driving power of the starter is small at a lowtemperature of the battery, causing greater frictions among movableparts and, hence, making the cranking of the automatic-start time of theengine 11 prone to variations, the basic valve lift quantity Bstop isset at a relatively large value in order to change the target valve liftquantity VLstop in a direction of stabilizing the cranking such as adirection of increasing the intake airflow at the automatic-start timeof the engine 11.

[0111] On the other hand, the valve lift quantity correction coefficientCstop is a correction coefficient, which is used for correcting thebasic valve lift quantity Bstop for the engine automatic-stop time whenthe performance of the battery deteriorates due to a large number ofoperations carried out so far to automatically start the engine 11. Avalve lift quantity correction coefficient Cstop is determined independence on the number of engine automatic stops carried out so far orthe number of engine automatic starts carried out so far by using aformula or the map of valve lift quantity correction coefficient Cstopshown in FIG. 7B.

[0112] In general, the larger the number of engine automatic stopscarried out so far or the number of engine automatic starts carried outso far, the larger the consumption of power from the battery and, hence,the lower the performance of the battery. Thus, the larger the number ofengine automatic stops carried out so far or the number of engineautomatic starts carried out so far, the smaller the driving power ofthe starter. As a result, as the number of engine automatic stopscarried out so far or the number of engine automatic starts carried outso far increases, the automatic start characteristic of the engine 11tends to deteriorate.

[0113] From the relation described above, the map of valve lift quantitycorrection coefficient Cstop shown in FIG. 7B is created so that, in azone where the number of engine automatic stops carried out so far orthe number of engine automatic starts carried out so far is greater thana predetermined value, that is, in a zone where the deterioration ofperformance of battery caused by the increased number of operationscarried out so far to automatically start the engine 11 cannot beignored, the larger the number of engine automatic stops carried out sofar or the number of engine automatic starts carried out so far, thelarger the value at which the valve lift quantity correction coefficientCstop is set. Thus, the larger the number of engine automatic stopscarried out so far or the number of engine automatic starts carried outso far, the smaller the driving power of the starter and, hence, themore the cranking at the automatic-start time of the engine 11 is proneto variations, the larger the value at which the valve lift quantitycorrection coefficient Cstop is set. A large valve lift quantitycorrection coefficient Cstop changes the target valve lift quantityVLstop in a direction of stabilizing the cranking or a direction ofincreasing the intake airflow. In a zone where the number of engineautomatic stops carried out so far or the number of engine automaticstarts carried out so far is smaller than the predetermined value, thatis, in a zone where the deterioration of performance of battery causedby the increased number of operations carried out so far toautomatically start the engine 11 can be almost ignored, on the otherhand, the valve lift quantity correction coefficient Cstop is set at1.0. In this zone, the target valve lift quantity VLstop is equal to thebasic valve lift quantity Bstop.

[0114] In the map of basic valve lift quantity Bstop shown in FIG. 7A,as temperature information for determining a temperature of the engine11 and/or a temperature of the battery, an engine cooling-watertemperature Tw is used. It is to be noted, however, that anothertemperature such as an intake air temperature Ti, an ambient temperatureTa or an oil temperature To can also be used as well. In a word, it ispreferable to find a basic valve lift quantity Bstop on the basis ofone, two or more pieces of such temperature information.

[0115] At the step 104, the basic valve lift quantity Bstop is correctedby multiplying the basic valve lift quantity Bstop by the valve liftquantity correction coefficient Cstop to find a target valve liftquantity VLstop for the automatic-stop time of the engine 11. However,the basic valve lift quantity Bstop can also be used as a target valvelift quantity VLstop for the automatic-stop time of the engine 11 as itis without correction of the basic valve lift quantity Bstop bymultiplying the basic valve lift quantity Bstop by the valve liftquantity correction coefficient Cstop.

[0116] After finding the target valve lift quantity VLstop for theautomatic-stop time of the engine 11, the flow of the program goes on toa step 105 at which variable-valve lift control is executed to controlthe variable-valve lift mechanism 30 of the intake valve 28 and thevariable-valve lift mechanism 31 of the exhaust valve 29 so that thevalve lift quantities of the intake valve 28 and the exhaust valve 29are adjusted to their respective target valve lift quantities VLstop.The processing of the steps 104 and 105 is carried out to play the roleof an automatic-stop-time valve control means.

[0117] The flow of the program goes on to a step 106 to determinewhether or not the valve lift quantities of the intake valve 28 and theexhaust valve 29 have each been adjusted to the target valve liftquantity VLstop. At a point of time the valve lift quantities of theintake valve 28 and the exhaust valve 29 become equal to theirrespective target valve lift quantities VLstop, the flow of the programgoes on to a step 107 at which conductions of currents to the drivingmotors of the variable-valve lift mechanisms 30 and 31 are halted.

[0118] By carrying out the processing described above, thevariable-valve lift mechanisms 30 and 31 are halted with the valve liftquantities of the intake valve 28 and the exhaust valve 29 set at theirrespective target valve lift quantities VLstop, which are each a valvelift quantity presumed to be suitable for the next automatic start or avalve lift quantity making it difficult for residual gas left in thecylinders to leak out.

[0119] It is to be noted that, in a system employing a variable valvetiming mechanism in conjunction with the variable-valve lift mechanisms30 and 31, when the engine 11 is automatically stopped, control can beexecuted to adjust the valve lift quantities to their respective targetvalve lift quantities for the automatic stop of the engine 11 and thevalve timings to their respective target valve timings for the automaticstop of the engine 11.

[0120] Automatic Start Control

[0121] The automatic-start control program represented by the flowchartshown in FIG. 8 is executed repeatedly at predetermined time intervalsduring an automatic stop of the engine 11. When this program is invoked,the flowchart begins with a step 201 to determine whether or notautomatic-start conditions are satisfied. In the case of amanual-transmission car (an MT car), typically, the automatic-startconditions include conditions (1) to (3) described as follows.

[0122] (1): The speed of the vehicle shall be 0 km/h. That is, thevehicle shall be in a stopped state.

[0123] (2): The clutch pedal shall be in a state of being depressed.

[0124] (3): The brake pedal shall be in a state of not being depressed.

[0125] If conditions (1) to (3) are all satisfied, the automatic-startconditions are considered to hold true. If even only one of conditions(1) to (3) is not satisfied, on the other hand, the automatic-startconditions are considered not to hold true.

[0126] It is to be noted that the automatic-start conditions can bemodified if necessary. In the case of an automatic-transmission car (anAT car), on the other hand, the automatic-start conditions are typicallyconsidered to be satisfied when the shift lever has been shifted to adrive range or the like with the brake pedal put in a state of beingdepressed. In a word, the automatic-start conditions are considered tobe satisfied when the driver has carried out operations in a preparationfor running the vehicle, be the vehicle an AT car or an MT car.

[0127] If the automatic-start conditions are satisfied in anautomatic-stop state of the engine 11, a request for an engine start isdetermined to exist. In this case, the flow of the program goes on to astep 202 at which target valve lift quantities VLstart of the intakevalve 28 and the exhaust valve 29 for the engine automatic-start timeare each computed in accordance with an equation given below. Asdescribed above, the target valve lift quantity VLstart is a valve liftquantity presumed to be optimum for the automatic-start time of theengine 11.

VLstart=Bstart×C1start×C2start

[0128] where reference notation Bstart is a basic valve lift quantityfor the engine automatic-start time whereas reference notations C1startand C2start are respectively first and second valve-lift-quantitycorrection coefficients for correcting the basic valve lift quantityBstart.

[0129] A basic valve lift quantity Bstart is set in dependence on anengine-cooling-water temperature Tw detected immediately before anautomatic start of the engine 11 by using a formula or a map preparedfor the basic valve lift quantity Bstart for the engine automatic-starttime like one shown in FIG. 9A.

[0130] In accordance with the basic valve lift quantity Bstart's mapshown in FIG. 9A, the lower the engine cooling-water temperature Tw usedas temperature information indicating the temperatures of the engine 11and the battery, the larger the value at which the basic valve liftquantity Bstart is set. Thus, in order to cope with the fact that thedriving power of the starter is small at a low temperature of the engine11 or the battery, causing greater frictions among movable parts and,hence, making the cranking of the automatic-start time of the engine 11prone to variations, the basic valve lift quantity Bstart is set at arelatively large value in order to change the target valve lift quantityVLstart in a direction of stabilizing the cranking such as a directionof increasing the intake airflow at the automatic-start time of theengine 11.

[0131] On the other hand, the first valve lift quantity correctioncoefficient C1start is a correction coefficient, which is used forcorrecting the basic valve lift quantity Bstart for the engineautomatic-start time when the performance of the battery deterioratesdue to a large number of operations carried out so far to automaticallystart the engine 11. A first valve lift quantity correction coefficientC1start is determined in dependence on the number of engine automaticstops carried out so far or the number of engine automatic startscarried out so far by using a formula or the map of first valve liftquantity correction coefficient C1start shown in FIG. 9B. In thefollowing description, the number of engine automatic stops carried outso far and the number of engine automatic starts carried out so far arealso referred to as an engine automatic stop count NS and an engineautomatic start count NR respectively. The map of first valve liftquantity correction coefficient C1start shown in FIG. 9B is created sothat, in a zone where the number of engine automatic stops carried outso far or the number of engine automatic starts carried out so far issmaller than a predetermined value, that is, in a zone where thedeterioration of performance of battery caused by the increased numberof operations carried out so far to automatically start the engine 11can be almost ignored, the first valve lift quantity correctioncoefficient C1start is set at 1.0. In a zone where the number of engineautomatic stops carried out so far or the number of engine automaticstarts carried out so far is greater than the predetermined value, thatis, in a zone where the deterioration of performance of battery causedby the increased number of operations carried out so far toautomatically start the engine 11 cannot be ignored, on the other hand,the larger the number of engine automatic stops carried out so far orthe number of engine automatic starts carried out so far, the larger thevalue at which the first valve lift quantity correction coefficientC1start is set.

[0132] In addition, the second valve lift quantity correctioncoefficient C2start is a correction coefficient, which is used forcorrecting the basic valve lift quantity Bstart for the engineautomatic-start time when the performance of the battery deterioratesdue to a long automatic-stop time Ts of the engine 11 or largeconsumption of power from the battery during the automatic stop of theengine 11. A second valve lift quantity correction coefficient C2startis determined in dependence on the automatic-stop time Ts of the engine11 by using a formula or the map of second valve lift quantitycorrection coefficient C2start shown in FIG. 9C. The map of second valvelift quantity correction coefficient C2start shown in FIG. 9C is createdso that, in a zone where the automatic-stop time Ts of the engine 11 issmaller than a predetermined value, that is, in a zone where thedeterioration of performance of battery caused by the large consumptionof power from the battery during the automatic stop of the engine 11 canbe almost ignored, the second valve lift quantity correction coefficientC2start is set at 1.0 meaning no correction of the basic valve liftquantity Bstart. In a zone where the automatic-stop time Ts of theengine 11 is greater than the predetermined value, that is, in a zonewhere the deterioration of performance of battery caused by the largeconsumption of power from the battery during the automatic stop of theengine 11 cannot be ignored, on the other hand, the longer theautomatic-stop time Ts of the engine 11, the larger the value at whichthe second valve lift quantity correction coefficient C2start is set.

[0133] When the number of engine automatic stops carried out so far orthe number of engine automatic starts carried out so far increases orwhen the automatic-stop time Ts of the engine 11 becomes long, thedriving power of the starter decreases, making the cranking of theautomatic-start time of the engine 11 prone to variations. In this case,the first valve lift quantity correction coefficient C1start or thesecond valve lift quantity correction coefficient C2start is set at alarge value, which changes the target valve lift quantity VLstart in adirection of stabilizing the cranking or a direction of increasing theintake airflow at the automatic start of the engine 11.

[0134] In the basic valve lift quantity Bstart map shown in FIG. 9A, astemperature information for determining a temperature of the engine 11and/or a temperature of the battery, an engine cooling-water temperatureTw is used. It is to be noted, however, that another temperature such asan intake air temperature Ti, an ambient temperature Ta or an oiltemperature To can also be used as well. In a word, it is preferable tofind a basic valve lift quantity Bstart on the basis of one, two or morepieces of such temperature information.

[0135] Then, at the step 202, the basic valve lift quantity Bstart iscorrected by multiplying the basic valve lift quantity Bstart by thefirst valve lift quantity correction coefficient C1start and the secondvalve lift quantity correction coefficient C2start to find a targetvalve lift quantity VLstart for the automatic-start time of the engine11. However, one of the first valve lift quantity correction coefficientC1start and the second valve lift quantity correction coefficientC2start or both can be eliminated from the formula for computing atarget valve lift quantity VLstart.

[0136] After finding the target valve lift quantity VLstart for theautomatic-start time of the engine 11, the flow of the program goes onto a step 203 at which variable-valve lift control is executed tocontrol the variable-valve lift mechanism 30 of the intake valve 28 andthe variable-valve lift mechanism 31 of the exhaust valve 29 so that thevalve lift quantities of the intake valve 28 and the exhaust valve 29are adjusted to their respective target valve lift quantities VLstart.The processing of the steps 202 and 203 is carried out to play the roleof an automatic-start-time valve control means.

[0137] The flow of the program goes on to a step 204 to determinewhether or not the valve lift quantities of the intake valve 28 and theexhaust valve 29 have been adjusted to their respective target valvelift quantities VLstart. At a point of time the valve lift quantities ofthe intake valve 28 and the exhaust valve 29 become equal to theirrespective target valve lift quantities VLstart, the flow of the programgoes on to a step 205 at which automatic start control is executed toturn on the starter and to start the engine 11 automatically. Theprocessing of the step 205 is carried out to play the role of anautomatic-start control means.

[0138] By carrying out the processing described above, the engine 11 isautomatically started with the valve lift quantities of the intake valve28 and the exhaust valve 29 set at their respective target valve liftquantities Vlstart.

[0139] It is to be noted that, in a system employing a variable valvetiming mechanism in conjunction with the variable-valve lift mechanisms30 and 31, when the engine 11 is automatically started, control can beexecuted to adjust the valve lift quantities to their respective targetvalve lift quantities for the automatic-start time of the engine 11 andthe valve timings to their respective target valve timings for theautomatic-start time of the engine 11.

[0140] Engine-Stall-Generation-Time Control

[0141] The engine-stall-generation-time control program represented bythe flowchart shown in FIG. 10 is executed repeatedly at predeterminedtime intervals after the start of automatic-start control. When thisprogram is invoked, the flowchart begins with a step 301 to determinewhether or not the engine so-called engine stall has been generated dueto a failure of an automatic start of the engine 11 by, typically,determining whether or not the engine speed NE has decreased to 0. If noengine stall has been generated, the execution of the program is endedwithout doing anything.

[0142] If an engine stall has been generated, on the other hand, theflow of the program goes on to a step 302 at which the target valve liftquantity VLstart set for the intake valve 28 to be used at anautomatic-start time of the engine 11 is increased by a predeterminedvalve lift quantity increment ΔVL in a correction process to increasethe intake airflow as follows:

VLstart=VLstart+ΔVL

[0143] A valve lift quantity increment ΔVL is determined in dependenceon the number of previous engine stalls by using a formula or the valvelift quantity increment ΔVL's map like one shown in FIG. 11. Inaccordance with the valve lift quantity increment ΔVL's map, the largerthe number of previous engine stalls, the larger the value at which avalve lift quantity increment ΔVL is set.

[0144] After correcting the target valve lift quantity VLstart for theautomatic-start time of the engine 11, the flow of the program goes onto a step 303 at which variable-valve lift control is executed tocontrol the variable-valve lift mechanism 30 of the intake valve 28 sothat the valve lift quantity of the intake valve 28 is adjusted to thecorrected target valve lift quantity VLstart.

[0145] The flow of the program goes on to a step 304 to determinewhether or not the valve lift quantities of the intake valve 28 and theexhaust valve 29 have been adjusted to their respective corrected targetvalve lift quantities VLstart. At a point of time the valve liftquantities of the intake valve 28 and the exhaust valve 29 become equalto their respective corrected target valve lift quantities VLstart, theflow of the program goes on to a step 305 at which automatic startcontrol is re-executed to automatically start the engine 11.

[0146] It is to be noted that, in a system employing a variable valvetiming mechanism in conjunction with the variable-valve lift mechanisms30 and 31, when an engine stall is generated, the target valve liftquantities for the automatic-start time of the engine 11 and the targetvalve timings for the automatic-start time of the engine 11 can each becorrected to increase the intake airflow.

[0147] FIGS. 12 to 14 show time charts for the programs represented bythe flowcharts shown in FIGS. 6, 8 and 10.

[0148] The time charts shown in FIG. 12 are time charts of typicalcontrol, which is executed when an engine cooling-water temperature Twis determined to be higher than a predetermined value and the catalyst23 is determined to have been activated. In this case, when theautomatic stop conditions are satisfied during an operation of theengine 11, the engine 11 is automatically stopped. Right after theengine 11 is automatically stopped, since the engine cooling-watertemperature Tw is determined to be higher than the predetermined valueand the catalyst 23 is determined to have been activated, the targetvalve lift quantities VLstop of the intake valve 28 and the exhaustvalve 29 for the engine automatic-stop time are set in accordance withan engine-cooling-water temperature Tw detected right after theautomatic stop and in accordance with other information in order toattach importance to the next automatic start characteristic or therestart characteristic of the engine 11. After the valve lift quantitiesVLstop for the intake valve 28 and the exhaust valve 29 are controlledto their respective target valve lift quantities VLstop for the engineautomatic-stop time, which have each been set at a valve lift quantitypresumed to be suitable for the next automatic start, the execution ofthe control of the variable-valve lift mechanisms 30 and 31 is ended. Inthis way, during the automatic stop of the engine 11, the variable-valvelift mechanisms 30 and 31 are halted with the valve lift quantities ofthe intake valve 28 and the exhaust valve 29 set at their respectivetarget valve lift quantities VLstop, which are each a valve liftquantity presumed to be suitable for the next automatic start.

[0149] When the automatic start conditions are satisfied during theautomatic stop of the engine 11, target valve lift quantities VLstart ofthe intake valve 28 and the exhaust valve 29 for the engineautomatic-start time are each set at a valve lift quantity presumed tobe optimum for the automatic start on the basis of anengine-cooling-water temperature Tw detected immediately before theautomatic start and on the basis of other information. Then, after thevalve lift quantities of the intake valve 28 and the exhaust valve 29are corrected from their respective target valve lift quantities VLstopfor the engine automatic-stop time to their respective target valve liftquantities VLstart for the engine automatic-start time, the engine 11 isautomatically started. The target valve lift quantities VLstop for theengine automatic-stop time are each a valve lift quantity presumed to besuitable for an automatic start. On the other hand, the target valvelift quantities VLstart for the engine automatic-start time are each avalve lift quantity optimum for an automatic start. In this way, at anautomatic-start time of the engine 11, the engine 11 can beautomatically started at a valve lift quantity suitable for theautomatic start. It is thus possible to improve the automatic-startcharacteristic of the engine 11 and reduce exhaust emissions at theautomatic-start time.

[0150] As described above, right after an automatic stop of the engine11, first of all, the valve lift quantities of the intake valve 28 andthe exhaust valve 29 are each set at a valve lift quantity presumed tobe suitable for a next automatic start. Then, right before the automaticstart of the engine 11, the valve lift quantities of the intake valve 28and the exhaust valve 29 are each corrected to a valve lift quantityoptimum for the automatic start. Thus, when the engine 11 isautomatically started, the magnitudes of corrections for correcting thevalve lift quantities are small so that the valve lift quantities of theintake valve 28 and the exhaust valve 29 can each be corrected to avalve lift quantity optimum for an automatic start in a short period oftime. In addition, even if the valve lift quantities each set during theautomatic stop of the engine 11 at a valve lift quantity presumed to besuitable for the next automatic start inevitably deviates from acondition optimum for the next automatic start due to the fact that thestate of the engine 11 and/or the state of the vehicle have greatlychanged during the automatic stop of the engine 11, the valve liftquantities of the intake valve 28 and the exhaust valve 29 can each becorrected to a valve lift quantity optimum for the automatic start whenthe engine 11 is automatically started.

[0151] On the other hand, the time charts shown in FIG. 13 are timecharts of typical control, which is executed when an enginecooling-water temperature Tw is determined to be lower than apredetermined value and the catalyst 23 is determined to have not beenactivated. In this case, when the automatic stop conditions aresatisfied during an operation of the engine 11, the engine 11 isautomatically stopped. Right after the engine 11 is automaticallystopped, since the engine cooling-water temperature Tw is determined tobe lower than the predetermined value and the catalyst 23 is determinedto have not been activated, the target valve lift quantities VLstop ofthe intake valve 28 and the exhaust valve 29 for the engineautomatic-stop time are set at a valve lift quantity such as 0 or aminimum value making it difficult for residual gas left in the cylindersto leak, out in order to attach importance to reduction of exhaustemissions generated during the automatic stop of the engine 11. Afterthe valve lift quantities VLstop for the intake valve 28 and the exhaustvalve 29 are controlled their respective target valve lift quantitiesVLstop for the engine automatic-stop time, which have each been set at avalve lift quantity making it difficult for residual gas left in thecylinders to leak out, the execution of the control of thevariable-valve lift mechanisms 30 and 31 is ended. In this way, duringthe automatic stop of the engine 11 with the catalyst 23 put in aninactivated state, the variable-valve lift mechanisms 30 and 31 arehalted with the valve lift quantities of the intake valve 28 and theexhaust valve 29 set at their respective target valve lift quantitiesVLstop, which have each been set at a valve lift quantity making itdifficult for residual gas left in the cylinders to leak out. Thus, whenthe catalyst 23 is still in an inactivated state, residual gas left inthe cylinders can be prevented from leaking out during the automaticstop of the engine 11 so that it is possible to reduce exhaust emissionsgenerated during the automatic stop of the engine 11.

[0152] When the automatic start conditions are satisfied during theautomatic stop of the engine 11, target valve lift quantities VLstart ofthe intake valve 28 and the exhaust valve 29 for the engineautomatic-start time are each set at a valve lift quantity presumed tobe optimum for the automatic start on the basis of anengine-cooling-water temperature Tw detected immediately before theautomatic start and on the basis of other information. Then, after thevalve lift quantities of the intake valve 28 and the exhaust valve 29are corrected from their respective target valve lift quantities VLstopfor the engine automatic-stop time to their respective target valve liftquantities VLstart for the engine automatic-start time, the engine 11 isautomatically started. The target valve lift quantities VLstop for theengine automatic-stop time are each a valve lift quantity presumed to besuitable for an automatic start. On the other hand, the target valvelift quantities VLstart for the engine automatic-start time are each avalve lift quantity optimum for the automatic start. In this way, at anautomatic-start time of the engine 11, the engine 11 can beautomatically started at a valve lift quantity suitable for theautomatic start. When the catalyst 23 is still in an inactivated state,it is thus possible to improve the automatic-start characteristic of theengine 11 while reducing exhaust emissions at the automatic-start time.

[0153] The time charts shown in FIG. 14 are time charts of typicalcontrol executed in the event of an engine stall caused by a failure ofan automatic start of the engine 11. In this case, in the event of anengine stall, the target valve lift quantity VLstart set for the intakevalve 28 to be used at an automatic-start time of the engine 11 isincreased by a predetermined valve lift quantity increment ΔVL in acorrection process to increase the intake airflow. Then, aftercorrecting the target valve lift quantity VLstart for theautomatic-start time of the engine 11, variable-valve lift control isexecuted to control the variable-valve lift mechanism 30 of the intakevalve 28 so that the valve lift quantity of the intake valve 28 isadjusted to the corrected target valve lift quantities VLstart. At apoint of time the valve lift quantity of the intake valve 28 becomesequal to the corrected target valve lift quantity VLstart, automaticstart control is re-executed to automatically start the engine 11.

[0154] Thus, even in the event of an engine stall caused by a failure ofan automatic start of the engine 11, at an automatic restart time,automatic start control of the engine 11 can be executed with a valvelift quantity corrected to a value greater than a valve lift quantity atthe time of the engine stall, that is, corrected in a direction ofincreasing the intake airflow. As a result, the engine stall can beprevented from being generated several times consecutively, and theengine can therefore be restarted successfully at an early time.

[0155] In addition, in this embodiment, by using at least one of piecesof temperature information for determining a temperature of the engine11 or the battery and pieces of information for determining performanceof the battery, a target valve lift quantity VLstop for an engineautomatic-stop time and a target valve lift quantity VLstart for anengine automatic-start time are found. The pieces of temperatureinformation include the temperature of the engine cooling water, thetemperature of intake air, the ambient temperature and the temperatureof the oil while the pieces of information for determining performanceof the battery include the number of engine automatic stops carried outso far or the number of engine automatic starts carried out so far. Asdescribed above, the target valve lift quantity VLstop for an engineautomatic-stop time is a valve lift quantity presumed to be suitable forthe next automatic start. Thus, in order to cope with the fact that theperformance of the battery is poor at a low temperature of the battery,decreasing the driving power of the starter, causing greater frictionsamong movable parts and, hence, making the cranking of theautomatic-start time of the engine 11 prone to variations, the targetvalve lift quantity is corrected in a direction of stabilizing thecranking such as a direction of increasing the intake airflow at theautomatic-start time of the engine 11.

[0156] As described above, in this embodiment, the target valve liftquantity VLstop for an engine automatic-stop time is changed from avalve lift quantity presumed to be suitable for the next automatic startto a valve lift quantity making it difficult for residual gas left inthe cylinders to leak out and vice versa in dependence on an activationstate of the catalyst 23 or a temperature of the engine cooling-water.It is to be noted, however, that the target valve lift quantity VLstopfor an engine automatic-stop time can also be fixed at a valve liftquantity presumed to be suitable for the next automatic start or a valvelift quantity making it difficult for residual gas left in the cylindersto leak out.

[0157] In addition, this embodiment executes both the control to adjustthe valve lift quantity to the target valve lift quantity VLstop for anengine automatic-stop time in an automatic stop of the engine 11 and thecontrol to adjust the valve lift quantity to the target valve liftquantity VLstart for an engine automatic-start time in an automaticstart of the engine 11. However, only one of them can also be executed.

[0158] Furthermore, this embodiment uses a stepping motor as a means fordriving the variable-valve lift mechanisms 30 and 31. However, as themeans for driving the variable-valve lift mechanisms 30 and 31, a meansother than the stepping motor can also be employed. Examples of theother means are an electromagnetic actuator and an oil-pressureactuator. As an alternative, by directly driving the intake valve and/orthe exhaust valve by using an electromagnetic actuator, valve operationcharacteristics can be changed. The valve operation characteristicsinclude the valve lift quantity and the valve timing.

[0159] Moreover, while this embodiment applies the present invention toa system for changing the operation characteristics of the intake valveand the exhaust valve, this embodiment may also apply the presentinvention to a system for changing the operation characteristics of theintake valve only.

[0160] Second Embodiment

[0161] Next, a second embodiment of the present invention is explained.The second embodiment has the same configuration as that shown inFIG. 1. In the case of the second embodiment, however, processingrepresented by a flowchart shown in FIG. 15 is carried out as asubstitute for the first embodiment's processing represented by theflowchart shown in FIG. 8. The other control processing of the firstembodiment is also carried out by the second embodiment.

[0162] An automatic-start control program stored in a ROM andrepresented by the flowchart shown in FIG. 15 is executed by the ECU 27to automatically start the engine 11 when predetermined automatic-startconditions are satisfied in an automatic-stop state of the engine 11with a timing shown in time charts of FIG. 17. Then, till the timelapsing since the completion of the automatic start of the engine 11exceeds a variable-valve control prohibition time KCAST, the valve liftquantities of the intake valve 28 and the exhaust valve 29 are fixed attheir respective target valve quantities for the automatic-start time,and the control of the intake airflow based on the intake thevariable-valve lift control is prohibited. Instead, the intake airflowis controlled by adjusting the opening of the throttle valve 15 in themean time.

[0163] The following description explains processing carried out by theECU 27 by execution of the automatic-start control program representedby the flowchart shown in FIG. 15. The automatic-start control programrepresented by the flowchart shown in FIG. 15 is executed repeatedly atpredetermined time intervals during an automatic stop of the engine 11.The processing carried out at the steps 201 to 205 is the same as thatcarried out at the steps 201 to 205 of the first embodiment.

[0164] After completion of the step 203, the flow of the program goes onto a step 204 to determine whether or not the valve lift quantities ofthe intake valve 28 and the exhaust valve 29 have been adjusted to theirrespective target valve lift quantities VLstart. At a point of time thevalve lift quantities of the intake valve 28 and the exhaust valve 29become equal to their respective target valve lift quantities VLstart,the flow of the program goes on to a step 205 at which automatic startcontrol is executed to turn on the starter and to start the engine 11automatically. The processing of the step 205 is carried out to play therole of an automatic-start control means.

[0165] At the next step 226, the valve lift quantities of the intakevalve 28 and the exhaust valve 29 are fixed at their respective targetvalve quantities for the automatic-start time after completion of theautomatic start of the engine 11. The control of the intake airflowbased on the intake the variable-valve lift control is prohibited.Instead, throttle-valve control is started to control the intake airflowby adjusting the opening of the throttle valve 15.

[0166] The flow of the program goes on to a step 227 to determinewhether or not the time CAST lapsing since the completion of theautomatic start of the engine 11 has exceeded the variable-valve controlprohibition time KCAST. The variable-valve control prohibition timeKCAST is set at a period of time it takes to stabilize the operatingstate to a certain degree. The time required to stabilize the operatingstate is a period of time lapsing since the completion of the automaticstart of the engine 11. This period includes complicated and muchvariable transient times. In this case, in order to make the processingsimple, the variable-valve control prohibition time KCAST can be set ata fixed value determined in advance. As an alternative, a variable-valvecontrol prohibition time KCAST can be determined by searching thevariable-valve control prohibition time KCAST's map like one shown inFIG. 16 for a particular value dependent on the number of automaticstops carried out so far since the start of the running state of thevehicle (that is, an automatic stop count NS) or the number of automaticstarts carried out so far since the start of the running state of thevehicle (that is, an automatic start count NR). That is, thevariable-valve control prohibition time KCAST is set at the particularvalue.

[0167] The larger the automatic stop count NS or the automatic startcount NR, the more frequently adverse effects such as deterioration ofthe drivability and deterioration of exhaust emissions are experienced.Such deteriorations are caused by the variable-valve lift control. Inaccordance with the map shown in FIG. 16, the larger the automatic stopcount NS or the automatic start count NR, the larger the value at whichthe variable-valve control prohibition time KCAST is set. Thus, thenumber of adverse effects caused by the variable-valve lift control canbe reduced. It is to be noted that, in accordance with the typical mapshown in FIG. 16, in a zone where the automatic stop count NS or theautomatic start count NR is smaller than a predetermined value A, thevariable-valve control prohibition time KCAST is set at a fixed value,which is a lower limit. In a zone where the automatic stop count NS orthe automatic start count NR is greater than another predetermined valueB, on the other hand, the variable-valve control prohibition time KCASTis set at another fixed value, which is an upper limit.

[0168] If the determination result obtained at the step 227 indicatesthat the time CAST lapsing since the completion of the automatic startof the engine 11 has not exceeded the variable-valve control prohibitiontime KCAST, the flow of the program goes back to the step 226. Theprocessing of the steps 226 and 227 is carried out to play the roles ofa variable-valve control prohibition means and a throttle-valve controlmeans.

[0169] At a point of time the determination result obtained at the step227 indicates that the time CAST lapsing since the completion of theautomatic start of the engine 11 has exceeded the variable-valve controlprohibition time KCAST, the flow of the program goes back to the step228 at which the variable-valve control is permitted and thethrottle-valve control is ended. In consequence, after the time CASTlapsing since the completion of the automatic start of the engine 11exceeds the variable-valve control prohibition time KCAST, the valvelift quantities of the intake valve 28 and the exhaust valve 29 arecontinuously changed in accordance with information such as anaccelerator position and an operating state of the engine 11 in order tocontrol the intake airflow. In the course of the intake-air-flow controlbased on the control of the variable-valve lift quantities, the throttlevalve 15 is fixed typically at a completely opened position to reducethe resistance of intake air.

[0170] It is to be noted that, in a system employing a variable valvetiming mechanism in conjunction with the variable-valve lift mechanisms30 and 31, before the time CAST lapsing since the completion of theautomatic start of the engine 11 exceeds the variable-valve controlprohibition time KCAST, the valve lift quantities can be fixed at theirrespective target valve lift quantities for the automatic-start time ofthe engine 11 and the valve timings can be fixed at their respectivetarget valve timings for the automatic-start time of the engine 11.

[0171] In the case of the embodiment described above, before the timeCAST lapsing since the completion of the automatic start of the engine11 exceeds the variable-valve control prohibition time KCAST, the valvelift quantities of the intake valve 28 and the exhaust valve 29 arefixed and the intake-air-flow control based on the control of thevariable-valve lift quantities is prohibited. Instead, the intakeairflow is controlled by adjusting the opening of the throttle valve 15.Thus, during a period including complicated and much variable transienttimes right after an automatic start of the engine 11, by using theconventional system, the field-proven throttle-valve control can beexecuted as the control of the intake airflow in order to adjust theintake airflow in a stable manner. As a result, after the automaticstart of the engine 11, deterioration of the drivability anddeterioration of exhaust emissions can be avoided.

[0172] In addition, in the case of this embodiment, after completion ofan automatic start of the engine 11, the valve lift quantities of theintake valve 28 and the exhaust valve 29 are fixed at their respectivetarget valve lift quantities for the automatic-start time of the engine11 or for a time prior to the completion of the automatic start of theengine 11. Thus, before and after the completion of the automatic startof the engine 11, the valve lift quantities can each be sustained at afixed value to eliminate variations in valve lift quantities. As aresult, a torque shock and/or deterioration of exhaust emissions can beprevented from occurring due to changes in valve lift quantities.

[0173] It is to be noted that the fixed values at which the valve liftquantities are sustained after the automatic start of the engine 11 donot have to be the target valve lift quantities for the automatic-starttime of the engine 11. Instead, the fixed values at which the valve liftquantities are sustained after the automatic start of the engine 11 canbe each a constant determined in advance or found by using a map, aformula or the like in dependence on operating states for theautomatic-start time of the engine 11, which include a temperature ofthe cooling water, a temperature of the oil, an ambient temperature andan automatic halt period of the engine 11.

[0174] In addition, in the case of this embodiment, a map used forfinding a variable-valve control prohibition time KCAST is created insuch a way that, the larger the automatic stop count NS or the automaticstart count NR, the larger the value at which the variable-valve controlprohibition time KCAST is set. After the start of a running state of thevehicle, the automatic stop count NS or the automatic start count NR issmall so that adverse effects such as deterioration of exhaust emissionsand deterioration of the drivability, which are caused by thevariable-valve lift control, are experienced less frequently. Thus,after the start of a running state of the vehicle, the variable-valvecontrol prohibition time KCAST is set at a small value so as to startthe variable-valve lift control from an early time after an automaticstart of the engine 11. By starting the variable-valve lift control froman early time, it is possible to let the improvement of the performancesuch as improvement of the fuel economy resulting from thevariable-valve lift control take precedence of others. Then, as theautomatic stop count NS or the automatic start count NR increases afterthe start of a running state of the vehicle so that the adverse effectscaused by the variable-valve lift control are experienced morefrequently, the variable-valve control prohibition time KCAST is set ata large value so as to let avoidance of the adverse effects caused bythe variable-valve lift control take precedence of the improvement ofthe performance by execution the variable-valve lift control.

[0175] As described above, this embodiment uses a stepping motor as ameans for driving the variable-valve lift mechanisms 30 and 31. It is tobe noted, however, that, as the means for driving the variable-valvelift mechanisms 30 and 31, a means other than the stepping motor canalso be employed. Examples of the other means are an electromagneticactuator and an oil-pressure actuator. As an alternative, by directlydriving the intake valve and/or the exhaust valve by using anelectromagnetic actuator, valve operation characteristics can bechanged. The valve operation characteristics include the valve liftquantity and the valve timing.

[0176] In addition, while this embodiment applies the present inventionto a system for changing the operation characteristics of the intakevalve and the exhaust valve, this embodiment may also apply the presentinvention to a system for changing the operation characteristics of theintake valve only.

[0177] Third Embodiment

[0178] Next, a third embodiment of the present invention is explained.The third embodiment has the same configuration as that shown in FIG. 1.In the case of the third embodiment, however, processing represented bya flowchart shown in FIG. 18 is carried out as a substitute for thefirst embodiment's processing represented by the flowchart shown in FIG.8. The other control processing of the first embodiment is also carriedout by the third embodiment.

[0179] An automatic-start control program stored in a ROM andrepresented by the flowchart shown in FIG. 18 is executed by the ECU 27to automatically start the engine 11 when predetermined automatic-startconditions are satisfied in an automatic-stop state of the engine 11. Itis to be noted that, at that time, the variable-valve lift mechanisms 30and 31 are each set at a position proper for a restart operation.

[0180] The ECU 27 executes the automatic-start control programrepresented by the flowchart shown in FIG. 18 to accompany an automaticstop of the engine 11. The automatic-start control program representedby the flowchart shown in FIG. 18 is executed repeatedly atpredetermined time intervals based on a count value of typically acounter not shown in the figure. When the program is invoked, theflowchart begins with a step 201 to determine whether or not theautomatic-start conditions are satisfied.

[0181] If a determination result obtained at the step 201 is anacknowledgement, the flow of the program goes on to a step 232. At thestep 232, the voltage VB of the battery 41 mounted on the vehicle iscompared with a voltage criterion value KVBAT.

[0182] The voltage criterion value KVBAT is a value set for thefollowing reason. If the voltage VB of the battery 41 is low so that avoltage applied to a stepping motor for rotating the control shaft 35 isnot sufficient, the responsiveness of the stepping motor deteriorateseven if valve lift control is executed on the basis of, among others, anoperating state. If the responsiveness of the stepping motordeteriorates, the rotation of the control shaft 35 is inevitably late,being incapable of following a target valve lift quantity. Thus, atarget intake airflow cannot be obtained. As a result, exhaust emissionsunavoidably worsen. For this reason, the voltage criterion value KVBATis set at a value to be used as a criterion for determining whether ornot the problem described above arises.

[0183] If a comparison result obtained at the step 232 indicates thatthe voltage VB of the battery 41 is equal to or higher than the voltagecriterion value KVBAT, the flow of the program goes on to a step 234 byway of a step 233. The processing of these steps is carried out toexecute variable-valve lift quantity control right after the restart ofthe engine 11. The variable-valve lift quantity control can be executedright after the restart of the engine 11 because the voltage VB of thebattery 41 is sufficiently high. Specifically, first of all, targetpositions of the intake and exhaust valves 28 and 29 are found at thestep 233. The target positions of the intake and exhaust valves 28 and29 are target valve lift positions of the intake and exhaust valves 28and 29 for the restart time of the engine 11. The target positions ofthe intake and exhaust valves 28 and 29 are found by using typically amap or a formula in dependence on operating states for the restart timeof the engine 11. The operating states include a temperature of thecooling water, a temperature of the oil, an ambient temperature and astop period of the engine 11.

[0184] After the target positions of the intake and exhaust valves 28and 29 are found, the variable-valve lift quantity control is executedat the step 234. Specifically, the variable-valve lift mechanism 30 ofthe intake valve 28 and the variable-valve lift mechanism 31 of theexhaust valve 29 are controlled so that the valve lift positions of theintake and exhaust valves 28 and 29 are brought to the target positionsof the intake and exhaust valves 28 and 29 for the restart time of theengine 11 before the execution of the program is ended.

[0185] If the comparison result obtained at the step 232 indicates thatthe voltage VB of the battery 41 is lower than the voltage criterionvalue KVBAT, on the other hand, the flow of the program goes on to astep 236 by way of a step 235. At the step 235, the variable-valve liftquantity control is prohibited before the flow of the program goes on tothe step 236. At the step 236, a target intake airflow is found by usingtypically a map or a formula in dependence on operating states for therestart time of the engine 11. The operating states include atemperature of the cooling water, a temperature of the oil, an ambienttemperature and a stop period of the engine 11. Then, control isexecuted to drive the throttle valve 15 so that the intake airflow intoa combustion chamber is brought to the target intake airflow.

[0186] As described above, if the voltage VB of the battery 41 is lowerthan the voltage criterion value KVBAT, the intake airflow control byusing the intake valve 28 is prohibited. Instead, control by using thethrottle valve 15 is executed.

[0187] Next, typical operations of the embodiment are explained byreferring to time charts shown in FIG. 19. An idle stop execution flagshown in a column (a) in FIG. 19 is a flag indicating an automatic stopoperation or a restart operation of the engine 11. First of all, at atime T1, the idle stop execution flag is turned on and the engine 11 isautomatically stopped by halting operations such as the fuel injectioncontrol and the ignition control. The engine speed NE decreases to 0 rpmat a time T2 as shown in a column (b) in FIG. 19. At the time T1, theopening of the throttle valve 15 is restored to a completely closedposition as shown in a column (d) in FIG. 19.

[0188] At the time T2, when the engine 11 is stopped as evidenced by anengine speed NE of 0 rpm, the variable-valve lift mechanism 30 is set totake the lift quantity of the intake valve 28 to a lift quantitysuitable for a restart of the engine 11 as shown in a column (d) in FIG.19. Then, at a time T3, when the lift quantity of the intake valve 28 isset at a position suitable for a restart of the engine 11, avariable-valve lift quantity control execution flag is set at an OFFstate as shown in a column (f) in FIG. 19.

[0189] Then, at a time T4, the engine 11 is automatically started at arequest made by the driver. For example, when a starter flag is turnedon as shown in a column (g) in FIG. 19, the idle stop execution flagshown in the column (a) in FIG. 19 is turned off to commence the restartoperation of the engine 11. In the case of this embodiment, if thevoltage VB of the battery 41 is lower than the voltage criterion valueKVBAT as shown in a column (c) in FIG. 19, the variable-valve liftquantity control of the intake valve 28 is prohibited and the intakeairflow control is executed by using the throttle valve 15 as shown in acolumn (e) in FIG. 19. Then, as the voltage VB of the battery 41 exceedsthe voltage criterion value KVBAT at a time T6, the throttle valve 15 isfixed at a predetermined opening and the variable-valve lift quantitycontrol of the intake valve 28 is executed. In this way, it is possibleto implement intake airflow control with good responsiveness.

[0190] As described above, if the voltage VB of the battery 41 is lowerthan the voltage criterion value KVBAT, the variable-valve lift quantitycontrol of the intake valve 28 is prohibited to inhibit the execution ofthe intake airflow control based on the variable-valve lift quantitycontrol. Thus, even if the precision of the variable-valve lift quantitycontrol becomes poor due to a low voltage VB of the battery 41,deteriorations of exhaust emissions can be suppressed because thevariable-valve lift mechanisms 30 and 31 are fixed.

[0191] Fourth Embodiment

[0192] Next, a fourth embodiment of the present invention is explained.The fourth embodiment has the same configuration as that shown inFIG. 1. In the case of the fourth embodiment, however, processingrepresented by a flowchart shown in FIG. 20 is carried out as asubstitute for the first embodiment's processing represented by theflowchart shown in FIG. 6. The other control processing of the firstembodiment is also carried out by the fourth embodiment.

[0193] The ECU 27 executes automatic stop control programs shown inFIGS. 20, 21, 24 and 25. In an operation to automatically stop theengine 11, the engine speed NE is gradually reduced as shown in timecharts of FIG. 26 in order to give no sense of incompatibility to thedriver. In order to gradually reduce the engine speed NE, it isnecessary to gradually decrease a torque output by the engine 11. Inorder to gradually decrease the torque output by the engine 11, it isnecessary to gradually reduce the input airflow QA and the fuelinjection volume TAU as shown in the same flowcharts. When the enginespeed NE is decreased to a resonant revolution speed zone, torque abruptreduction control is executed to abruptly decrease the torque output bythe engine 11. The torque abrupt reduction control is executed byabruptly reducing the input airflow QA and ending the injection of fuelby adjustment of the variable-valve mechanism 30 and the throttle valve15. By executing the torque abrupt reduction control at the time theengine speed NE is decreased to the resonant revolution speed zone, theengine speed NE can pass through the resonant revolution speed zone in ashort period of time. The resonant revolution speed zone is the enginespeed NE's zone in which the vibration of the engine 11 is resonant withthe vibration of the vehicle-driving system. The resonant revolutionspeed zone is typically the revolution speed range 300 to 400 rpm. FIG.27 shows a graph representing a relation between the engine speed NE andthe magnitude of a noise.

[0194] The following description explains processing carried out by theECU 27 by execution of the programs.

[0195] Automatic Stop Control

[0196] The automatic stop control program represented by the flowchartshown in FIG. 20 is executed repeatedly at predetermined intervals aftera request for a stop of the engine 11 is made when predeterminedautomatic stop control conditions are satisfied during an operation ofthe engine 11. The program is executed to play the role of an automaticstop control means. When the program is invoked, the flowchart beginswith a step 141 to determine whether or not the engine 11 is in a stateprior to an automatic stop process of the engine 11 or prior tocompletion of the an automatic stop process of the engine 11 bytypically determining whether or not the engine speed NE is higher thana criterion value for the completion of the automatic stop. If theengine 11 is in an automatic stop process of the engine 11, the flow ofthe program goes on to a step 142 to determine whether or not the enginespeed NE is in the resonant revolution speed zone or even lower than thezone. A criterion range used in the determination of the step 142 can bemade greater than the resonant revolution speed zone to a certain degreein order to provide a small margin to the determination. In a word, thecriterion range needs to include a resonant revolution speed, whichincreases the amplitude of vibration of the engine 11, the amplitude ofvibration of the vehicle-driving system and the magnitude of a noisewhen the frequency of the vibration of the engine 11 matches thecharacteristic frequency of the vibration of the vehicle-driving system.

[0197] If a determination result obtained at the step 142 indicates thatthe engine speed NE has not decreased to a value in the resonantrevolution speed zone, torque gradual reduction control is executed atsteps 143 and 144. The torque gradual reduction control begins with thestep 143 at which an intake airflow gradual reduction control programrepresented by the flowchart shown in FIG. 21 is executed to graduallyreduce the intake airflow QA. Then, at the next step 144, a fuelinjection volume gradual reduction control program represented by theflowchart shown in FIG. 24 is executed to gradually reduce the fuelinjection volume TAU. In this way, the torque output by the engine 11can be gradually decreased to gradually reduce the engine speed NEwithout providing a sense of incompatibility to the driver.

[0198] If a determination result obtained at the step 142 in a laterexecution of the automatic stop control program represented by theflowchart shown in FIG. 20 indicates that the engine speed NE hasdecreased to a value in the resonant revolution speed zone or a valuelower than the zone, on the other hand, torque abrupt reduction controlis executed at steps 145 and 146. The torque abrupt reduction controlbegins with the step 145 at which an intake airflow abrupt reductioncontrol program represented by the flowchart shown in FIG. 25 isexecuted to abruptly reduce the intake airflow QA. Then, at the nextstep 146, injection of fuel is ended. In this way, the torque output bythe engine 11 can be abruptly decreased to abruptly reduce the enginespeed NE so that the engine speed NE can pass through resonantrevolution speed zone in a short period of time.

[0199] Intake Airflow Gradual Reduction Control

[0200] When the intake airflow gradual reduction control programrepresented by the flowchart shown in FIG. 21 is invoked at the step 143of the flowchart shown in FIG. 20, the flowchart shown in FIG. 21 beginswith a step 143 a at which a target intake airflow gradual reductionquantity FQA is found for the present engine speed NE and the presentintake airflow QA by using a formula or a map prepared for the targetintake airflow gradual reduction quantity FQA as shown in FIG. 22. Themap of target intake airflow gradual reduction quantity FQA shown inFIG. 22 is created so that, the lower the engine speed NE, the smallerthe target intake airflow gradual reduction quantity FQA and, thesmaller the intake airflow QA, the smaller the target intake airflowgradual reduction quantity FQA.

[0201] After a target intake airflow gradual reduction quantity FQA isfound, the flow of the program goes on to a step 143 b at which a targetvalve lift quantity VL of the intake valve 28 is found for the presentengine speed NE and a target intake airflow by using a formula or a mapprepared for the target valve lift quantity VL of the intake valve 28 asshown in FIG. 23. The target intake airflow is a difference between thepresent intake airflow QA and the target intake airflow gradualreduction quantity FQA. The target valve lift quantity VL's map shown inFIG. 23 is created so that, the lower the engine speed NE, the smallerthe target valve lift quantity VL and, the smaller the target intakeairflow, that is, the smaller the difference between the present intakeairflow QA and the target intake airflow gradual reduction quantity FQA,the smaller the target valve lift quantity VL.

[0202] After the target valve lift quantity VL is computed, the flow ofthe program goes on to a step 143 c at which the variable-valve controlis executed to control the variable-valve lift mechanism 30 of theintake valve 28 so as to take the valve lift quantity of the intakevalve 28 to the target valve lift quantity VL.

[0203] It is to be noted that, in a system employing a variable valvetiming mechanism in conjunction with the variable-valve lift mechanism30, at a step 202, a target valve lift quantity VL and a target valvetiming VT are computed. Then, at the next step 203, the variable-valvelift mechanism 30 of the intake valve 28 is controlled so as to take thevalve lift quantity of the intake valve 28 to the target valve liftquantity VL and the variable valve timing mechanism of the intake valve28 can be controlled so as to take the variable valve timing of theintake valve 28 to the target variable valve timing VT.

[0204] By carrying the processing described above repeatedly, thevariable-valve lift mechanism 30 of the intake valve 28 or both thevariable-valve lift mechanism 30 and the variable valve timing mechanismof the intake valve 28 are controlled so as to gradually reduce theinput airflow QA by the target intake airflow gradual reduction quantityFQA at one time.

[0205] Fuel Injection Volume Gradual Reduction Control

[0206] When the fuel injection volume gradual reduction control programshown in FIG. 24 is invoked at the step 144 of the flowchart shown inFIG. 20, a fuel injection volume TAU that takes the air-fuel ratio to atarget air-fuel ratio A/F is computed by using the present intakeairflow QA and the target air-fuel ratio A/F, which is typically set atthe stoichiometric air-fuel ratio. Thus, when the intake airflow gradualreduction control program represented by the flowchart shown in FIG. 21is executed to gradually reduce the intake airflow, the fuel injectionvolume TAU is also gradually reduced while the air-fuel ratio is beingsustained at the target air-fuel ratio A/F, which is typically thestoichiometric air-fuel ratio, so that the torque output by the engine11 and, hence, the engine speed NE are gradually reduced.

[0207] Intake Airflow Abrupt Reduction Control

[0208] When the fuel injection volume abrupt reduction control programshown in FIG. 25 is invoked at the step 145 of the flowchart shown inFIG. 20, first of all, at a step 145 a, the target valve lift quantityVL of the intake valve 28 is set at a minimum value (>0). Then, at thenext step 145 b, the variable-valve control is executed to control thevariable-valve lift mechanism 30 of the intake valve 28 so as to takethe valve lift quantity of the intake valve 28 to the target valve liftquantity VL, which has been set at the minimum value.

[0209] It is to be noted that, in a system employing a variable valvetiming mechanism in conjunction with the variable-valve lift mechanism30, at a step 145 a, a target valve lift quantity VL and a target valvetiming VT that minimize the intake airflow QA are computed. Then, at thenext step 145 b, the variable-valve lift mechanism 30 of the intakevalve 28 is controlled so as to take the valve lift quantity of theintake valve 28 to the target valve lift quantity VL and the variablevalve timing mechanism of the intake valve 28 can be controlled so as totake the variable valve timing of the intake valve 28 to the targetvariable valve timing VT.

[0210] Then, the flow of the program goes on to a step 145 c at whichthe target throttle opening of the throttle valve 15 is set at 0 tocompletely close the throttle valve 15. Subsequently, at the next step145 d, throttle-valve control is executed to adjust the throttle valve15 so as to take the throttle opening to the target throttle opening ofthe throttle valve 15, which has been set at 0 to completely close thethrottle valve 15. By carrying out the above processing, the intakeairflow QA can be reduced abruptly.

[0211] In the case of the embodiment described above, in the intake airquantity control based on the variable-valve control, attention paid tothe fact that the responsiveness of the intake air quantity control isimproved without incurring a response delay of an air system leads toabrupt reduction of the intake airflow QA by execution of thevariable-valve control at the time the engine speed NE decreases to theresonant revolution speed area in the course of an operation toautomatically stop the engine 11. The air system starts from thethrottle valve 15 and ends at the cylinders. Thus, with a timing of theengine speed NE decreasing to the resonant revolution speed area, theintake airflow QA into a cylinder can be decreased abruptly with goodresponsiveness so that the engine speed NE can also be abruptlydecreased in the resonant revolution speed area. Thus, at an executiontime of the automatic stop control, the engine speed NE can pass throughthe resonant revolution speed area in a short period of time. As aresult, it is possible to reduce the amplitude of vibration and themagnitude of a noise, which are caused by the resonance phenomenon, aswell as make the driver feel no sense of incompatibility.

[0212] In addition, in the case of this embodiment, at an execution timeof torque abrupt reduction control, the variable-valve lift mechanism 30is controlled to establish a valve operation characteristic minimizingthe intake airflow QA, and the throttle valve 15 is completely closed.An example of the valve operation characteristic minimizing the intakeairflow QA is a state in which the valve lift quantity is equal to aminimum value. Thus, both the variable-valve control and thethrottle-valve control can be effectively utilized to set the intakeairflow QA into a cylinder at 0 quickly and, hence, reduce the outputtorque abruptly. By adopting this control technique, even in a systemincapable of controlling the intake valve 28 to a completely closedstate, the resonant revolution speed area can be passed through in ashort period of time so that it is possible to reduce the amplitude ofvibration and the magnitude of a noise, which are caused by theresonance phenomenon.

[0213] Furthermore, in the case of this embodiment, injection of fuel ishalted at an execution time of the torque abrupt reduction control.Thus, both the abrupt reduction of the intake airflow and thetermination of the fuel injection can effectively decrease the enginespeed abruptly.

[0214] Moreover, in the case of this embodiment, the fuel injectionvolume is adjusted so as to take the air-fuel ratio to a target air-fuelratio A/F at an execution time of the torque gradual reduction control.Thus, at the execution time of the torque gradual reduction control, theair-fuel ratio can be sustained at the target air-fuel ratio A/F. As aresult, the engine speed can be reduced gradually without deterioratingexhaust emissions.

[0215] In addition, in the case of this embodiment, the intake airflowQA is gradually decreased by execution of the variable-valve control atan execution time of the torque gradual reduction control. Thus, theintake airflow QA into a cylinder can be gradually reduced with goodresponsiveness at the execution time of the torque gradual reductioncontrol in order to decrease the output torque gradually with a highdegree or reliability.

[0216] It is to be noted that the torque gradual reduction controlraises a small problem of a response delay incurred in the air system incomparison with the torque abrupt reduction control. Thus, the intakeairflow QA can be gradually reduced by executing only the torque gradualreduction control. It is needless to say, nevertheless that, at anexecution time of the torque gradual reduction control, both thevariable-valve control and the throttle-valve control can be executed toreduce the intake airflow QA gradually.

[0217] Moreover, this embodiment has a configuration wherein thevariable-valve lift mechanism 30 cannot be controlled to put the intakevalve 28 in a completely closed state, that is, a state with a valvelift quantity of 0. In the case of a system having a variable-valve liftmechanism controllable to put the intake valve 28 in a completely closedstate, however, the variable-valve lift mechanism can be controlled toput the intake valve 28 in a completely closed state, that is, a statewith a valve lift quantity of 0, at an execution time of the torqueabrupt reduction control. The intake airflow QA into a cylinder can beset at 0 instantaneously to abruptly reduce the engine speed at anexecution time of the torque abrupt reduction control. Thus, theresonant revolution speed area can be passed through in a short periodof time so that it is possible to substantially reduce the amplitude ofvibration and the magnitude of a noise, which are caused by theresonance phenomenon.

[0218] It is to be noted that, in the case of this embodiment, in asmall lift mode, the position of the control shaft 35 is set so as toset a point of contact with the link arm 34 at the position of theeccentric cam 36, that is, a position at a shortest distance from theaxial center of the control shaft 35 as shown in FIG. 4. For this smalllift mode, the curvature of the bottom surface range of the pressure cam39, that is, the bottom surface range in contact with the roller 40, isdesigned into a curvature at which the pressure cam 39 does not bend theroller 40 downward. Thus, in the small lift mode, the pressure cam 39never bends the roller 40 downward even when the cam 37 of the cam shaft32 shifts the reciprocating cam 38 horizontally. As a result, the liftquantity of the intake valve 28 can be set at 0. In such aconfiguration, by setting the variable-valve lift mechanism 30 in thesmall lift mode when the engine speed NE passes through the resonantrevolution speed area in an operation to automatically stop the engine11, the intake airflow can be set at 0 so that the resonant revolutionspeed area can be passed through in a short period of time.

[0219] Furthermore, in the case of this embodiment, the throttle valve15 is provided on the intake pipe 12. However, the throttle valve 15 canbe eliminated and the intake airflow can be controlled by using only thevariable-valve mechanism.

[0220] In addition, in the case of this embodiment, a stepping motor isused as a means for driving the variable-valve lift mechanism 30.However, as the means for driving the variable-valve lift mechanism 30,a means other than the stepping motor can also be employed. Examples ofthe other means are an electromagnetic actuator and an oil-pressureactuator. As an alternative, by directly driving the intake valve and/orthe exhaust valve by using an electromagnetic actuator, valve operationcharacteristics can be changed. The valve operation characteristicsinclude the valve lift quantity and the valve timing.

[0221] Moreover, while this embodiment applies the present invention toa system for changing the operation characteristics of the intake valveand the exhaust valve, this embodiment may also apply the presentinvention to a system for changing the operation characteristics of theintake valve only.

[0222] Furthermore, the scope of the present invention is not limited toa vehicle run by only a driving power output by the engine. Instead, thepresent invention can also be applied to a hybrid car run by both adriving power output by the engine and a driving power output by adriving-power source other than the engine. An example of the otherdriving-power source is a motor.

[0223] Although the present invention has been described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications will be apparent to those skilled in the art. Such changesand modifications are to be understood as being included within thescope of the present invention as defined in the appended claims.

What is claimed is:
 1. A control apparatus for an internal combustionengine, comprising: variable-valve control means for controlling atleast an intake airflow by adjusting valve operation characteristics ofthe intake valve, exhaust valve or both the intake and exhaust valves ofthe engine; an automatic-stop control means, which is used forautomatically stopping the engine when a predetermined automatic stopcondition is satisfied in the course of an operation of the engine; andan automatic-stop valve control means, which is used for computingtarget valve operation characteristics for an automatic-stop time on thebasis of present states of the engine and/or a vehicle employing theengine and for controlling the valve operation characteristics to thetarget valve operation characteristics for an automatic-stop time rightafter the automatic-stop control means automatically stops the engine.2. A control apparatus for an internal combustion engine according toclaim 1, further comprising: an automatic-start control means, which isused for automatically starting the engine when a predeterminedautomatic-start condition is satisfied in an automatic-stop state of theengine; and an automatic-start valve control means, which is used forcomputing target valve operation characteristics for an automatic-starttime on the basis of present states of the engine and/or the vehicle andfor controlling the valve operation characteristics to the target valveoperation characteristics for an automatic-start time when theautomatic-start control means automatically starts the engine.
 3. Acontrol apparatus for an internal combustion engine according to claim1, wherein the automatic-stop valve control means computes valveoperation characteristics presumed to be proper for the next automaticstart as the target valve operation characteristics for anautomatic-stop time.
 4. A control apparatus for an internal combustionengine according to claim 1, wherein the automatic-stop valve controlmeans computes valve operation characteristics, which sets valve liftquantities at 0 or minimum values, as the target valve operationcharacteristics for an automatic-stop time.
 5. A control apparatus foran internal combustion engine according to claim 1, wherein theautomatic-stop valve control means computes the target valve operationcharacteristics for an automatic-stop time on the basis of at least oneof an automatic-stop count, a cooling-water temperature, an intake-airtemperature, an oil temperature and information having correlations withany one of the automatic-stop count, the cooling-water temperature, theintake-air temperature, and the oil temperature.
 6. A control apparatusfor an internal combustion engine, comprising: a variable-valve controlmeans for controlling at least an intake airflow by adjusting valveoperation characteristics of the intake valve, exhaust valve or both theintake and exhaust valves of the engine; an automatic-start controlmeans, which is used for automatically starting the engine when apredetermined automatic-start condition is satisfied in anautomatic-stop state of the engine; and an automatic-start valve controlmeans, which is used for computing target valve operationcharacteristics for an automatic-start time on the basis of presentstates of the engine and/or a vehicle employing the engine and forcontrolling the valve operation characteristics to the target valveoperation characteristics for an automatic-start time when theautomatic-start control means automatically starts the engine.
 7. Acontrol apparatus for an internal combustion engine according to claim6, wherein the automatic-start valve control means computes the targetvalve operation characteristics for an automatic-start time on the basisof at least one of an automatic-stop count, an automatic-stop time, acooling-water temperature, an intake-air temperature, an oil temperatureand information having correlations with the automatic-stop count, theautomatic-stop time, the cooling-water temperature and the oiltemperature.
 8. A control apparatus for an internal combustion engineaccording to claim 6, wherein, in the event of an engine stall caused bya failure of an automatic start of the engine, the automatic-start valvecontrol means controls the valve operation characteristics to increasethe intake airflow and the automatic-start control means restarts theengine.
 9. A control apparatus for an internal combustion engine,comprising: a variable-valve control means for controlling at least anintake airflow by adjusting valve operation characteristics of theintake valve, exhaust valve or both the intake and exhaust valves of theengine; an automatic-start control means, which is used forautomatically starting the engine when a predetermined automatic-startcondition is satisfied in an automatic-stop state of the engine; avariable-valve control prohibition means for fixing the valve operationcharacteristics at predetermined conditions during a variable-valvecontrol prohibition period, which is a predetermined period after theautomatic start of the engine carried out by the automatic-start controlmeans; and a throttle-valve control means for controlling the intakeairflow during the variable-valve control prohibition period byadjusting the opening of a throttle valve provided on an intake path ofthe engine.
 10. A control apparatus for an internal combustion engineaccording to claim 9, wherein, during the variable-valve controlprohibition period, the variable-valve control prohibition means fixesthe valve operation characteristics each at a target valve operationcharacteristic for an automatic start of the engine.
 11. A controlapparatus for an internal combustion engine according to claim 9,wherein the variable-valve control prohibition means sets thevariable-valve control prohibition period at a value dependent on theautomatic-stop count or automatic-start count of the engine.
 12. Acontrol apparatus for an internal combustion engine, comprising: avariable-valve mechanism for varying valve operation characteristics ofthe intake valve, exhaust valve or both the intake and exhaust valves ofthe engine in control of an intake airflow by; and an automatic-stopcontrol means, which is used for adjusting the intake airflow bycontrolling the variable-valve mechanism and/or controlling a throttlevalve so as to gradually reduce a torque output by the engine and stopthe engine when a predetermined automatic-condition is satisfied duringan operation of the engine, wherein, in a process to gradually reduce atorque output by the engine and stop the engine, the automatic-stopcontrol means executes torque abrupt reduction control to abruptlyreduce the intake airflow by controlling the variable-valve mechanism soas to abruptly reduce the torque output by the engine with a timing withwhich an engine speed is about to pass through a predeterminedrevolution speed range.
 13. A control apparatus for an internalcombustion engine according to claim 12, wherein the predeterminedrevolution speed range is set to include a resonant revolution speedzone in which vibration of the engine is resonant with vibration of avehicle-driving system.
 14. A control apparatus for an internalcombustion engine according to claim 12, wherein the automatic-stopcontrol means controls the variable-valve mechanism so as to put theintake valve in a completely closed state during the torque abruptreduction control.
 15. A control apparatus for an internal combustionengine according to claim 12, wherein the automatic-stop control meanscontrols the variable-valve mechanism and completely closes the throttlevalve so as to minimize the intake airflow during the torque abruptreduction control.
 16. A control apparatus for an internal combustionengine according to claim 12, wherein the automatic-stop control meansterminates injection of fuel during the torque abrupt reduction control.17. A control apparatus for an internal combustion engine according toclaim 12, wherein, in a process to gradually reduce a torque output bythe engine and stop the engine, the automatic-stop control meanscontrols a fuel injection volume so as to sustain an air-fuel ratio at atarget air-fuel ratio till the engine speed is reduced to thepredetermined revolution speed range.
 18. A control apparatus for aninternal combustion engine, comprising: a variable-valve control meansfor by varying valve operation characteristics of the intake valveand/or exhaust valve of the engine in accordance with a voltage outputby a battery mounted on a vehicle employing the engine and controllingat least an intake airflow into a combustion chamber of the engine; anautomatic-start control means, which is used for automatically startingthe engine when a predetermined automatic-restart condition is satisfiedin an automatic-stop state of the engine; a battery-voltage detectionmeans for detecting a voltage output by the battery; and avariable-valve control prohibition means, which is used for prohibitingvariable control executed by the variable-valve control means on theintake valve and/or the exhaust valve in dependence on the voltage ofthe battery detected by the battery-voltage detection means after theengine is automatically started by the automatic-start control means.19. A control apparatus for an internal combustion engine according toclaim 18, wherein the variable-valve control prohibition means prohibitsthe variable control executed by the variable-valve control means on theintake valve and/or the exhaust valve till the voltage of the batterydetected by the battery-voltage detection means reaches a predeterminedlevel.
 20. A control apparatus for an internal combustion engineaccording to claim 18, wherein the variable-valve control means is ameans for setting valve lift quantities of the intake valve and/or theexhaust valve by utilizing a voltage output by the battery.